It creeps in slowly in the dark of night, and once inside, it almost never goes away.
It's known as the Hum, a steady, droning sound that's heard in places as disparate as Taos, N.M.; Bristol, England; and Largs, Scotland.
But what causes the Hum, and why it only affects a small percentage of the population in certain areas, remain a mystery, despite a number of scientific investigations. (The Top 10 Unexplained Phenomena)
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.
The cases seem to have several factors in common: Generally, the Hum is only heard indoors, and it's louder at night than during the day. It's also more common in rural or suburban environments; reports of a hum are rare in urban areas, probably because of the steady background noise in crowded cities.
Who hears the Hum?
Only about 2 percent of the people living in any given Hum-prone area can hear the sound, and most of them are ages 55 to 70, according to a 2003 study by acoustical consultant Geoff Leventhall of Surrey, England.
Most of the people who hear the Hum (sometimes referred to as "hearers" or "hummers") describe the sound as similar to a diesel engine idling nearby. And the Hum has driven virtually every one of them to the point of despair. (Video: Listen to 6 Spooky Sounds)
"It's a kind of torture; sometimes, you just want to scream," retiree Katie Jacques of Leeds, England, told the BBC. Leeds is one of several places in Great Britain where the Hum has recently appeared.
"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."
Being dismissed as crackpots or whiners only exacerbates the distress for these complainants, most of whom have perfectly normal hearing. Sufferers complain of headaches, nausea, dizziness, nosebleeds and sleep disturbances. At least one suicide in the United Kingdom has been blamed on the Hum, the BBC reports. (The Top 10 Spooky Sleep Disorders)
The Hum zones Bristol, England, was one of the first places on Earth where the Hum was reported. In the 1970s, about 800 people in the coastal city reported hearing a steady thrumming sound, which was eventually blamed on vehicular traffic and local factories working 24-hour shifts.
Another famous hum occurs near Taos, N.M. Starting in spring 1991, residents of the area complained of a low-level rumbling noise. 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.
Windsor, Ontario, is another Hum hotspot. Researchers from the University of Windsor and Western University in London, Ontario, were recently given a grant to analyze the Windsor Hum and determine its cause.
Researchers also have been investigating the Hum in Bondi, a seaside area of Sydney, Australia, for several years, to no avail. "It sends people around here crazy - all you can do is put music on to block it out. Some people leave fans on," one resident told the Daily Telegraph.
Back in the United States, the Kokomo Hum was isolated in a 2003 study financed by the Indiana city's municipal government. 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.