Hey, Bob, do you hear that?
The Taos Hum is an as-yet unexplained, far-reaching phenomenon involving a very low frequency sound heard by an incredibly small number of people, most notably around Taos, New Mexico, where one study suggested that around 2% of the local population have reported experiencing the mysterious hum.1 Centers of hum activity include the Southwestern, Northeastern, and Northwestern United States, the United Kingdom (where it, or something very similar in nature, was referred to as the "Bristol Hum" as early as 1977), and parts of Germany and Sweden.
Remarkably, while hearers are scattered across the globe, and have had very little contact with each other until the advent of the internet, virtually every hearer describes the hum in the exact same way: A large diesel engine idling nearby. Most hearers report that they began to hear the hum abruptly at some point in their life (one man saying that it was as if it was suddenly "switched on" in his head, like a radio), and that from this point forward they continue to hear this grinding, pulsing sound for the rest of their lives.
Humming away
The sound itself is very enigmatic in nature and often varies in intensity, from being completely inaudible at times to becoming so intense that hearers report blurred vision, dizziness, fatigue, nosebleeds, inability to sleep, headaches, and all kinds of other undesirable side effects. Long term exposure to the hum has reportedly caused insomnia, marital strife (as usually only one partner will hear the hum), and various forms of paranoia and craziness that I can't even begin to diagnose.2 The hearers often report that the hum 'blinks' on and off, like morse code, leading many to believe that the hum is intelligently modulated. It is not uncommon for hearers to report not hearing the hum for days, only to have it return in full force once again.
Interestingly, hearers may find specific locations where they do not hear the hum; this could be as close as the park across the street or as far away as Korea or China, where some traveling hum-hearers have reported no hum. The hum can be blocked by ambient acoustic noises; many sleep with music or noise machines to cancel out the effects of the hum. Earplugs, reportedly, do not block the sound. One man claimed a 10 - 15% increase in loudness after descending to the bottom of a 650 foot mineshaft in Michigan.
Out of tune
It is generally believed that the hum is not acoustic in nature, but is rather some sort of low-frequency pulsed electrical signal, the origin of which is unknown. Why some people can detect this hum and others appear oblivious to it is the source of constant debate. Additionally, whether there is one "global" hum or whether the hum is composite of various hums of different sources is a contested point.
A predominant belief is that the hum is a result of an extremely low frequency military communications tool, such as the ELF system used by the United States Navy to communicate with their nuclear submarine fleet. The residents of Taos, New Mexico managed to alert their congressman (Bill Richardson D-NM) and after a "thorough investigation" it was determined that while there was no clear source of the hum, the government certaintly didn't have anything to do with it, and that all of the top secret military bases near Taos were pure coincidence.3
A man in Sweden by the name of Stefan Lofstrand claims that the hum is caused by a massive, top-secret, underground military tunneling operation that has been operating around his rural home for years. He has reportedly called the military and asked them to stop, but they also feign innocence.
Other theories include: more government programs that may or may not exist, aliens from outer space, industrial machinery, mass hysteria, strange geological activity, schumann resonance, or a particularly intense case of Tinnitus.
An interesting footnote is the "Sausalito Hum", a similar phenomenon affecting coastal areas of Sausalito, California. After a series of tests, it was determined that the mysterious "hum" was caused by a particularly noisy species of fish (porichthys notatus), colloquially known as the California singing fish, which tend to make lots of noise at night during their mating season.4
1. I believe that this figure is inflated due to the hysteria and publicity surrounding the study.
2. There are reports that a man in England was driven to suicide by the hum, but I haven't found any real evidence supporting this.
3. http://homepages.tesco.net/~John.Dawes2/taos.htm
4. http://fha-eng.com/hum.htm