A mysterious hum that has been described as a steady droning, throbbing or rumbling has been heard in the United States and as far away as Bristol, England and Largs, Scotland.

Residents described a low-level rumbling noise. Los Alamos National Laboratory researchers, as well as experts from the University of New Mexico and Sandia National Laboratories could identify no origin.

Katie Jacques of Leeds, England told the BBC "It's a kind of torture; sometimes, you just want to scream. It's worst at night. 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."

Researchers don't think the "hearers" are "hearing things." They don't blame mass hysteria or hypochondria. They also don't think it caused by aliens beaming waves to earth from flying saucers. For the most part they believe it is caused by industrial equipment. 

Science has no answer to a mysterious hum has been plaguing people in all corners of the earth. Theorist of the mysterious hum have studied medical causes, submarine messages and natural, man-made and alien origins.  It is listed in "The Top 10 Unexplained Phenomena."For the most part, the hum is louder at night, mainly heard indoors and is more common in rural or suburban environments. Although the sound of the city may drown it out.

A Daimler Chrysler plant is believed to be the cause of one outbreak of the mysterious hum. In 2003, the municipal government of Kokomo, Indiana, financed a study to find the cause of the Kokomo Hum. Investigators pointed to two industrial site that were producing noise at a specific frequency. The manufacturers tried to block the sounds, but residents continued to complain about the noise.

In spite of many scientific investigations, the cause of the mysterious hum and why it only plagues a small group of people is unknown.

A study by Geoff Leventhall, an acoustical consultant in Surrey, England found the hum is only heard by about 2 percent of people living in areas where it is reported. Most of the "Hearers" as they call themselves, are between the ages of 55 and 70. Sufferers complain of headaches, nausea, dizziness, nosebleeds and sleep disturbances. The hum sounds vaguely like an idling engine and has been described as torture and, according to "The Top 10 Spooky Sleep Disorders" has been blamed for at least once suicide in England.