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.

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.

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.

[link to www.nbcnews.com]