Mysterious, low-frequency hums heard all around the world have for decades baffled scientists and driven residents mad with frustration. So what causes the Hum heard in places such as Taos, N.M., Bristol, England, and Largs, Scotland?

We’re still not entirely sure, researchers say.

“Reports started trickling in during the 1950s from people who had never heard anything unusual before; suddenly, they were bedeviled by an annoying, low-frequency humming, throbbing or rumbling sound,” NBC News reminds us.

Although the Hum is slightly different in each area, there are some common factors: the sound is generally only heard indoors and usually becomes more powerful at night. The sound is also usually restricted to remote, non-urban areas:

It gets weirder: in areas that supposedly have the Hum, only two percent of the population say they can hear it. And the “hearers” (or “hummers”) are usually in the 55 to 77-year-old range, according to a 2003 study by acoustical consultant Geoff Leventhall of Surrey, England.

The “hearers,” who have become increasingly frustrated and desperate over the years, describe the sound as being similar to the humming of an idling diesel engine.

“It’s a kind of torture; sometimes, you just want to scream,” Katie Jacques of Leeds, England, told the BBC.

Leeds has in recent years experienced the Hum phenomenon.

“It’s worst at night,” Jacques said. “It’s hard to get off to sleep because I hear this throbbing sound in the background. … You’re tossing and turning, and you get more and more agitated about it.”

And it’s not just irritation that has “hearers” desperate – some claim the Hum is responsible for headaches, nausea, dizziness, nosebleeds and “sleep disturbances,” the NBC News report notes.

In fact, according to the BBC, at least one suicide has been blamed on the mysterious noise.

Reports say the Hum first appeared in Bristol, England, in the 1970s. Roughly 800 people at the time reported hearing a low, droning noise that officials claimed was a mixture of traffic and factory activity.

And then the Hum started appearing all over the place.

“Another famous hum occurs near Taos, N.M. Starting in spring 1991, residents of the area complained of a low-level rumbling noise,” NBC News reports.

Here’s supposed audio of the Hum in Taos, New Mexico:

“A team of researchers from Los Alamos National Laboratory, the University of New Mexico, Sandia National Laboratories and other regional experts were unable to identify the source of the sound,” the report adds

Likewise, a team of researchers from the University of Windsor and Western University in London, Ontario continue to investigate a Hum in Windsor, Ontario.

Elsewhere, researchers investigate a Hum in Bondi, Australia – but with no luck or explanation.

“It sends people around here crazy — all you can do is put music on to block it out. Some people leave fans on,” one Bondi resident told the Daily Telegraph.

Indiana is dealing with its own Hum-related issues.

“[T]he Kokomo Hum was isolated in a 2003 study financed by the Indiana city’s municipal government,” NBC says.

“The investigation revealed that two industrial sites — one a Daimler Chrysler plant — were producing noise at specific frequencies. Despite noise-abatement measures, some residents continue to complain of the Hum,” it adds.

There’s one thing researchers agree on: the Hum is real and not just the product of someone’s imagination. The area where researchers disagree, however, is on the cause of the Hum.

Geoff Leventhall was able to trace the source of the Kokomo Hum to a building’s heating unit.

There’s some evidence that the Hum generates from high-pressure gas lines, electrical power lines, and/or wireless communication devices. But this isn’t definite and only in a few cases has the sound been traced back to electronic devices.

Further confusing the issue is the fact that most “hearers” have normal to perfect hearing. So it’s not an issue on that end.

Environmental issues may be responsible, the NBC News reports adds, “including seismic activity such as microseisms — very faint, low-frequency earth tremors that can be generated by the action of ocean waves.”

“Other hypotheses, including military experiments and submarine communications, have yet to bear any fruit,” the report adds.

Leventhall doesn’t expect to crack the mystery any time soon.

“It’s been a mystery for 40 years, so it may well remain one for a lot longer,” he told the BBC.

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