The Taos Hum

The Taos Hum is a faint, low-frequency humming noise heard in and near the town of Taos, New Mexico. Not only is the hum's source a mystery, but its peculiar qualities are as well: only about 2 percent of Taos residents - about 1,400 people - can hear it. The low hum - between 30 and 80 Hz on the frequency scale - has been described by hearers as sounding like a diesel engine idling in the distance or having a slow beat-note sound. Some people perceive it as being louder indoors than outdoors. More mysterious still, some hearers who are bothered by the sound have tried earplugs and other acoustic quieting devices to block it out - to no effect. Investigations by scientists, including some from the prestigious Sandia National Laboratories, have failed to find a source or even a plausible explanation for the phenomenon. One theory is that the source is the U.S. Navy's ELF (extra-low frequency) communications system that is used to communicate with its submarine fleet. The Navy, of course, accepts no such responsibility.
If you'd like to hear the Taos Hum, here are some recordings in .wav format (click icon to listen):

Taos isn't the only town afflicted with an annoying hum. According to The Taos Hum Homepage, "Nearly every state in the U.S. has at least one 'hum hearer' report, including Alaska and Hawaii. The largest number of reports come from the southwestern U.S., the Pacific Northwest, and southeastern states. Worldwide, the hum has caused such problems in the U.K. and Sweden that hum-hearer support groups have formed there. There are hum-hearer reports from Italy and from Mexico." The Bristol Hum is the most widely reported hum in the U.K.
A long list of hum reports from around the U.S. can be read here, and you can even add your own to the list.