For some, the noise is irritating but manageable. For others, the low-frequency sound can cause physical discomfort or illness and may also feel like a vibration moving through the body.

The hum can be difficult to detect outdoors. It is more commonly noticed inside buildings, especially at night when people are trying to sleep. Someone may look outside for a nearby engine or machine, only to find nothing that could explain the noise.

Even more puzzling, other people in the same room may hear nothing at all.

Reports of The Hum Begin in England

The phenomenon first gained widespread attention in Bristol, England, during the mid-1970s. The Bristol Evening Post began receiving a stream of letters from residents who described hearing an unexplained sound and wanted to know what was producing it.

One proposed explanation pointed to large industrial fans operating inside a department store warehouse. Yet the reports continued even after the warehouse closed several years later.

Similar accounts later emerged elsewhere in the United Kingdom, particularly in coastal communities including Hythe, Plymouth, Southampton and Swansea. Reports also came from London.

The mysterious sound became known as The Hum phenomenon, or simply The Hum.

During the 1990s, reports began appearing in the United States, initially in Taos, New Mexico and Kokomo, Indiana. The phenomenon has since been documented in Canada, Australia, New Zealand, South Africa and several European cities. Most reports tend to come from relatively densely populated areas.

According to the Norwegian Broadcasting Corporation (NRK), residents around Oslo also reported an unexplained humming noise a couple of years ago.

Canadian teacher Glen MacPherson first heard The Hum while living and working on the west coast of Canada. After moving to another city in the same region, he could no longer hear it.

His curiosity eventually led him to establish the interactive The World Hum Map and Database Project in 2012. The project gathers reports and location data from people who say they have experienced the sound.

Possible Sources Range From Machines to Nature

Researchers and the public have proposed numerous explanations for The Hum. Suggested causes include acoustic pollution from human activity, naturally occurring environmental sounds and conspiracy theories involving the CIA or even aliens.

Many technologies can generate low-frequency noise, including ventilation systems, heat pumps, road traffic and windmills. Nature also produces sounds in this range through sources such as ocean waves striking the shore and wind moving across the landscape.

The mystery has drawn the attention of hearing specialists and audiology researchers around the world. One of them is Markus Drexl, a professor at the Norwegian University of Science and Technology (NTNU).

Drexl, two PhD research fellows and a postdoc studied 28 people in Germany who reported hearing an unexplained hum or buzz.

Testing Whether the Hum Comes From Outside

The researchers examined two main hypotheses.

The first was that The Hum might be an externally produced sound that could be measured. Such noise could come from industry, infrastructure or natural processes that generate low-frequency waves.

“We know that there are people who hear low-frequency sounds that can actually be measured, even if other people don’t hear them. But it’s not so easy to find the source of these sound waves, because it’s a struggle to localize low-frequency sounds,” Drexl said.

Low-frequency sound waves have long wavelengths, which allows them to travel across considerable distances. That makes their source especially difficult to pinpoint.

Most Participants Did Not Have Exceptional Hearing

The team first investigated whether the participants were unusually sensitive to known low-frequency sounds.

Most showed no exceptional ability in this range. Only two people had better than average hearing at certain low frequencies.

“Even though the group we tested was small, it still means that the hypothesis of having especially good hearing for low-frequency sounds does not hold for most people,” Drexl said.

However, he noted an important limitation. Tiny variations in hearing thresholds (microstructures) may allow some people to detect sound within an extremely narrow frequency range, such as between 50 and 51 Hertz. Standard hearing tests are not designed to capture differences this precise.

Could the Inner Ear Be Producing the Noise?

The cochlea inside the inner ear naturally creates faint sounds at different frequencies, generally between about 500 and 5000 Hertz. These noises do not serve a direct purpose. Instead, they are a by-product of the ear’s physiological sound amplification process.

“Most of us don’t hear these sounds. However, a few people can actually hear the sounds that the ear itself produces. And these sounds can be measured objectively,” Drexl said.

Known as oto-acoustic emissions, these sounds can be detected by placing a sensitive microphone inside the ear canal. In certain individuals, spontaneous oto-acoustic emissions may be perceived as distressing tinnitus.

“One hypothesis was that the participants in our group could hear oto-acoustic emissions at low frequencies. That’s why we tested whether they had them,” says Drexl.

But… the answer was no.

Low-Frequency Tinnitus May Explain Many Cases

“Then there are people who hear something that cannot be measured objectively. We believe people in this category have a form of low-frequency tinnitus,” Drexl said.

Tinnitus, often described as ringing in the ears, occurs when someone perceives a sound inside the ear or head even though no external source is producing it.

Tinnitus may be temporary or persistent. People often initially interpret the sound as something coming from their surroundings.

When the noise continues after they change locations, however, they may eventually realize that it is not being generated by anything nearby.

Based on current knowledge of hearing and the results of the participant tests, Drexl believes the most likely explanation has two parts.

A small number of people who hear The Hum may genuinely have unusually sensitive low-frequency hearing. For most, however, the experience may be a type of tinnitus in which the sound begins within the auditory system.

“Based on our results, although we haven’t ruled out cases of physical external sound sources, we suggest that subjective tinnitus in the low-frequency range is often the cause of hearing pulsations of low-frequency sound perceptions,” he said.

Scientists Need a Better Picture of Low-Frequency Hearing

Drexl became interested in The Hum through his research into low-frequency sound.

“What we know about the hearing system is mainly based on how we capture and process sound with higher frequencies. We know less about how the auditory system handles and processes low-frequency sound, or infrasound,” he said.

Concern about noise from technological sources has increased during the past decade, particularly for sounds in the low-frequency range (between about 20 and 250 Hz) and infrasound (below 20 Hz).

“If we want to conduct a thorough assessment of low-frequency sounds and infrasound, we first need a better understanding of how sensory systems process low-frequency sound and infrasound,” he said.

New findings from the Texas A&M College of Veterinary Medicine and Biomedical Sciences (VMBS) point to one possible answer. Researchers found that certain compounds in coffee may activate NR4A1, a receptor that is becoming increasingly important in studies of aging, stress responses and disease.

The research, recently published in Nutrients, offers one of the first direct links between coffee compounds and NR4A1. That connection may help explain some of the broad health effects associated with drinking coffee.

“Coffee has well-known health-promoting properties,” said Dr. Stephen Safe, distinguished professor and Sid Kyle Endowed Chair in Veterinary Toxicology in VMBS’ Department of Veterinary Physiology and Pharmacology. “What we’ve shown is that some of those effects may be linked to how coffee compounds interact with this receptor, which is involved in protecting the body from stress-induced damage.”

How NR4A1 Helps Protect the Body

NR4A1 is part of a group of nuclear receptors that help control gene activity when the body is exposed to stress or tissue damage.

In earlier research, Safe and his collaborators described NR4A1 as a “nutrient sensor,” meaning that it can respond to dietary compounds and contribute to the body’s ability to remain healthy with age.

“If you damage almost any tissue, NR4A1 responds to bring that damage down,” Safe said. “If you take that receptor away, the damage is worse.”

Studies have connected NR4A1 with inflammation, metabolism and tissue repair. Each of these processes is closely involved in age-related conditions, including cancer, neurodegenerative disease and metabolic disorders.

A Possible Mechanism Behind Coffee’s Benefits

Large observational studies have linked coffee consumption with a reduced risk of Alzheimer’s disease, Parkinson’s disease and metabolic disease. However, those studies have generally shown associations rather than explaining exactly how coffee might produce protective effects.

Safe and his team proposed that NR4A1 could be one part of that explanation.

The project involved researchers from across Texas A&M, including Dr. Robert Chapkin, Dr. Roger Norton, Dr. James Cai and Dr. Shoshana Eitan. Their work helped show coffee’s protective effects in neurological models.

The researchers found that several compounds in coffee can bind to NR4A1 and alter its activity. The most active included polyhydroxy and polyphenolic compounds such as caffeic acid.

“What we’re saying is that at least part of coffee’s health benefits may come through binding and activating this receptor,” Safe said.

In laboratory models, these compounds also changed cell behavior in ways associated with disease protection. They reduced cellular damage and slowed the growth of cancer cells.

When the researchers removed NR4A1 from the cells, those protective effects disappeared. That result provided additional evidence that the receptor helps mediate at least some of coffee’s biological effects.

Coffee’s Benefits May Extend Beyond Caffeine

Caffeine is the largest individual component in coffee, but the study indicates that it may not be the main source of the beverage’s protective effects.

Instead, naturally occurring compounds that are also present in many fruits and vegetables appeared to have a stronger influence on NR4A1.

“Caffeine binds the receptor, but it doesn’t do much in our models,” Safe said. “The polyhydroxy and polyphenolic compounds are much more active.”

This finding may help explain why large population studies have linked both caffeinated and decaffeinated coffee with similar health benefits.

One Pathway Among Many

Safe cautioned that coffee is chemically complex and probably affects the body through multiple biological routes.

“There are many receptors and many mechanisms involved,” he said. “What we’re showing is that this could be one of the important pathways.”

The study was designed to investigate biological mechanisms. It does not establish direct cause and effect in people or prove that drinking coffee prevents disease.

“There’s still a lot of work to be done,” Safe said. “We’ve made the connection, but we need to better understand how important that connection is.”

The results support a growing body of research showing that diet, especially plant-based compounds, can influence biological pathways involved in aging and disease.

Because NR4A1 plays a role in several medical conditions, the findings may also contribute to future drug development. Safe’s team is studying synthetic compounds that target the receptor more effectively than natural dietary substances, with the aim of developing possible treatments for cancer and other diseases.

The work also highlights the potential importance of routine dietary choices.

“Coffee is a very complex mixture of compounds,” Safe said. “It’s a very potent combination.”

What the Findings Mean for Coffee Drinkers

The research does not change current recommendations for coffee consumption. People can also respond differently depending on their health, sensitivity to caffeine and other individual factors.

Still, the findings provide scientists with something that has been difficult to identify: a possible biological explanation for coffee’s long-standing association with better health and longevity.

“I think it helps explain why coffee has the effects that it does,” Safe said. “It’s not just an observation — there’s a mechanism behind it.”

Researchers found that fluvoxamine (sold under the brand name Luvox), an inexpensive medication already commonly used to treat depression and other conditions, reduced fatigue and improved quality of life in adults with long COVID. The randomized, placebo-controlled trial was published in the Annals of Internal Medicine.

A Potential Treatment for Long COVID Fatigue

Fatigue is among the most frequent and disabling symptoms reported by people with long COVID. For some, the exhaustion is severe enough to interfere with employment, family responsibilities, and everyday activities. Despite the scale of the problem, there are still few treatments supported by strong clinical evidence.

“This is an important step forward for patients who have been desperate for evidence-based options,” says Edward Mills, senior author, professor in McMaster’s Department of Health Research Methods, Evidence, and Impact, and co-principal investigator of the trial. “Fluvoxamine showed consistent and meaningful benefits, and because it’s already widely used and well understood, it has clear potential for clinical use.”

Researchers from Canada, Brazil, and the United States jointly led the study. Clinical sites were located in Belo Horizonte and throughout Minas Gerais, Brazil.

The REVIVE-TOGETHER trial brought together investigators from McMaster University, the University of British Columbia, Stanford University, the University of Pittsburgh, Duke University, Georgetown University, and several Brazilian institutions.

Testing Fluvoxamine and Metformin

The trial included 399 adults in Brazil who had experienced ongoing fatigue for at least 90 days after a confirmed SARS-CoV-2 infection. Participants were randomly placed into one of three groups and received fluvoxamine (sold under the brand name Luvox), metformin (a common diabetes medication), or a placebo for 60 days.

“We wanted to test whether two existing, widely available, and affordable medications could help. Both had biological reasons to think they might work against long COVID fatigue, but neither had been rigorously tested for this purpose in a proper clinical trial,” says Mills.

Fluvoxamine performed better than the placebo in reducing fatigue. The statistical analysis indicated a 99 percent probability that the medication was more effective than the placebo. Participants who received fluvoxamine also reported gains in overall quality of life across several measures.

Metformin did not produce the same results. Earlier research found that taking metformin during the initial stage of a COVID infection could lower the risk of later developing long COVID. In this trial, however, the drug provided no meaningful improvement for people who already had established long COVID fatigue.

An Adaptive Clinical Trial Design

Researchers used a Bayesian adaptive trial design, which allowed them to end individual treatment groups early once the evidence became sufficiently clear. This approach can produce reliable conclusions more quickly than a conventional trial while maintaining scientific rigor.

“The trial used a sophisticated adaptive design that allowed it to reach conclusions more efficiently than traditional trials, stopping early when the evidence was clear enough – a design innovation as important as the findings themselves,” says Gilmar Reis, lead author, researcher with Cardresearch, a Brazilian clinical research center based in Belo Horizonte. Reis is also a part-time associate professor at McMaster.

More Research Is Still Needed

Long COVID continues to pose a major global health challenge and is estimated to affect about 65 million people worldwide. Because proven therapies remain scarce, most medical recommendations focus on supportive strategies, including activity pacing and management of individual symptoms.

The researchers caution that fluvoxamine is not a complete solution for long COVID. The condition can involve many different symptoms and biological processes, and the medication appears specifically promising for fatigue management.

Additional studies will be needed to determine which patients are most likely to benefit, understand why the drug works, and explore whether it could be used alongside other developing treatments.

“This trial gives clinicians their first strong evidence for a medication that helps reduce long COVID fatigue. Patients want something they can try today – and this finding brings us closer to that reality,” says Jamie Forrest, corresponding author and postdoctoral research fellow at the University of British Columbia.

The research was funded by The Latona Foundation.

The study followed nearly 13,000 adults and examined seven sweeteners that contain little or no calories. People who consumed the largest total amounts showed a more rapid decline in cognitive abilities than those who consumed the smallest amounts. The association was especially strong among people with diabetes.

The results do not show that sweeteners directly cause cognitive decline. They reveal an association, meaning other factors could help explain the pattern.

Seven Common Sweeteners Examined

Researchers studied aspartame, saccharin, acesulfame K, erythritol, xylitol, sorbitol and tagatose.

These ingredients are frequently added to ultra processed products, including flavored water, soft drinks, energy drinks, yogurt and desserts marketed as low in calories. Several are also sold separately for use in coffee, tea, cooking or baking.

“Low- and no-calorie sweeteners are often seen as a healthy alternative to sugar, however our findings suggest certain sweeteners may have negative effects on brain health over time,” said study author Claudia Kimie Suemoto, MD, PhD, of the University of São Paulo in Brazil.

Tracking Brain Health for Eight Years

The research included 12,772 adults living across Brazil. Participants were 52 years old on average and were monitored for approximately eight years.

At the beginning of the study, participants completed detailed food questionnaires describing what they had eaten and drunk during the previous year. Researchers then placed them into three groups according to their total sweetener intake.

People in the lowest consumption group averaged 20 milligrams per day (mg/day), while those in the highest group averaged 191 mg/day. In the case of aspartame, the amount consumed by the highest group was roughly equal to the aspartame in one can of diet soda.

Sorbitol was consumed in the largest quantity of any individual sweetener, with an average daily intake of 64 mg/day.

Participants completed cognitive assessments at the beginning, midpoint and end of the study. The tests measured several aspects of brain function, including verbal fluency, working memory, word recall and processing speed.

Verbal fluency refers to the ability to quickly retrieve and produce words. Working memory is the brain’s short term system for holding and using information, while processing speed reflects how quickly a person can understand and respond to information.

Higher Intake Linked to Faster Cognitive Decline

After accounting for age, sex, high blood pressure, cardiovascular disease and other relevant factors, the researchers found a clear difference between the intake groups.

People who consumed the greatest amounts of sweeteners experienced a 62% faster decline in overall thinking and memory abilities than those who consumed the least. Researchers estimated that this difference was comparable to about 1.6 additional years of aging.

Participants in the middle consumption group experienced a decline that was 35% faster than the decline observed in the lowest group. That difference was comparable to approximately 1.3 years of aging.

Stronger Association in Adults Under 60

Age appeared to influence the results. Among participants younger than 60, those who consumed the most sweeteners experienced faster declines in verbal fluency and overall cognitive performance than those who consumed the least.

Researchers did not find the same association among participants older than 60.

The connection between sweetener intake and faster cognitive decline was also stronger among people with diabetes than among those without the condition. People with diabetes may use sugar substitutes more frequently because they are often advised to limit products that rapidly raise blood sugar.

Six Sweeteners Linked to Memory Changes

When the researchers examined the sweeteners separately, six were associated with faster declines in overall cognition, particularly memory.

Those sweeteners were aspartame, saccharin, acesulfame K, erythritol, sorbitol and xylitol.

Tagatose was the only sweetener in the study that was not linked to cognitive decline.

“While we found links to cognitive decline for middle-aged people both with and without diabetes, people with diabetes are more likely to use artificial sweeteners as sugar substitutes,” Suemoto said. “More research is needed to confirm our findings and to investigate if other refined sugar alternatives, such as applesauce, honey, maple syrup or coconut sugar, may be effective alternatives.”

Important Limits of the Research

The study did not include every artificial sweetener currently used in food and beverages, so the findings cannot be applied to all sugar substitutes.

The dietary information was also provided by the participants themselves. Because people may forget foods or misjudge how much they consumed, self reported diet data can be imperfect.

Most importantly, the study was observational. It identified a relationship between higher sweetener consumption and faster cognitive decline, but it could not establish that the sweeteners caused those changes.

Key Findings

• The study followed 12,772 adults with an average age of 52.
• Researchers examined seven sweeteners commonly found in flavored water, soda, energy drinks, yogurt, low calorie desserts and other ultra processed foods.
• Participants who consumed the largest total amounts experienced faster declines in overall thinking and memory skills than those who consumed the smallest amounts.
• The difference was comparable to approximately 1.6 additional years of aging.
• The association appeared in adults younger than 60 but was not detected in those older than 60.
• The findings show a link, but they do not prove that sugar substitutes cause cognitive decline.

The research was supported by the Brazilian Ministry of Health, the Ministry of Science, Technology, and Innovation, and the National Council for Scientific and Technological Development.