Live Science 
July 27, 2013
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.
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.