"The Hum" is a generic name for a common class of phenomena consisting of a persistent, invasive low-frequency sound from an unknown source. In addition, the phenomena is only audible to certain people, existing at the far low end of the audio spectrum, with a peak power spectral density between about 10 and 60 Hz. The Hum is generally difficult to pick with microphones, though it has been on many occasions, and its source is difficult to localize. There has been some limited scientific research into The Hum, and its existence is well accepted. It is said to be heard loudest at night, and can cause great distress among "Hum sufferers."
Some locations that have reported this noise include Bristol, England, the Big Island of Hawaii (where its source is thought to be subterranean volcanic activity), the North Shore area of Auckland, in Taos, New Mexico, and several other locations. A specific Hum is usually named after the area where it is heard, i.e., the "Taos Hum" or the "Auckland Hum." The sound of The Hum is often described as that of a idling diesel engine in the distance, even though no such engine can ever be found. It may be audible in two locations dozens of miles apart, but usually it is localized in a relatively small area.
Various explanations have been advanced for The Hum. In at least two locations, a mysterious, persistent low-frequency hum has been investigated and then attributed to a known source: in Kokomo, India, where the sound was traced to a cooling tower emitting a 36 Hz tone and an air compressor intake emitting a 10 Hz tone, and on the Big Island of Hawaii, where a tone was attributed to known volcanic activity. On the Big Island, the native islanders point out the sound is more easily heard by men, which makes sense, considering that the larger the animal, the more attuned it is to lower frequencies, and men are known to be able to hear lower frequency sounds than women.
Regardless of a few instances where this noise has been explained, more often than not the source remains unknown even after significant investigation. Once the obvious sources of sound (traffic, household appliances, etc.) are eliminated, more exotic potential causes are often forwarded, including sounds from distant industrial sources, infrasound from geologic events, a microwave auditory effect from industrial pulsed microwaves emitters, tinnitus (a constant ringing in the ears), spontaneous otoacoustic emissions (a subtle noise emitted by the human ear itself), and, more speculatively, ionospheric heating projects like HAARP in Alaska.