The Hum is a phenomenon, or collection of phenomena, involving widespread reports of a persistent and invasive low-frequency humming, rumbling, or droning noise not audible to all people. Hums have been widely reported by national media in the UK and the United States. The Hum is sometimes prefixed with the name of a locality where the problem has been particularly publicized: e.g., the "Bristol Hum" or the "Taos Hum". It is unclear whether it is a single phenomenon; different causes have been attributed. In some cases, it may be a manifestation of tinnitus.
The essential element that defines the Hum is what is perceived as a persistent low-frequency sound, often described as being comparable to that of a distant diesel engine idling, or to some similar low-pitched sound for which obvious sources (e.g., household appliances, traffic noise, etc.) have been ruled out. There are a number of audio reproductions of the Hum available on the web, as well as at least one purported recording.
Other elements seem to be significantly associated with the Hum, being reported by an important proportion of hearers, but not by all of them. Some people hear the Hum only, or much more, inside buildings as compared with outdoors. Some perceive vibrations that can be felt through the body. Earplugs are reported as not decreasing it. For those who can hear the Hum, it can be a very disturbing phenomenon and it has been linked to at least three suicides in the UK  and one attempted suicide in the US.
A study into the Taos Hum indicated that at least two percent could hear it; each hearer at a different frequency between 32 Hz and 80 Hz, modulated from 0.5 to 2 Hz. Similar results have been found in an earlier British study. It seems to be possible for hearers to move away from it, with one hearer of the Taos Hum reporting its range was 48 km. There are approximately equal percentages of male and female hearers. Age does appear to be a factor, with middle aged people being more likely to hear it.:43
In 2006, Tom Moir, then of Massey University in Auckland, New Zealand, believes he has made several recordings of the Auckland Hum. His previous research using simulated sounds had indicated that the hum was around 56 hertz. The Taos Hum was between 32 and 80 hertz.
There is skepticism as to whether it exists as a physical sound. In 2009, the head of audiology at Addenbrooke's Hospital in Cambridge, David Baguley, said he believed people's problems with the hum were based on the physical world about one-third of the time, and stemmed from people focusing too keenly on innocuous background sounds the other two-thirds of the time. His current research focuses on using psychology and relaxation techniques to minimise distress, which can lead to a quieting or even removal of the noise.
Geoff Leventhall, a noise and vibration expert, has suggested cognitive behavioral therapy may be effective in helping those affected. "It's a question of whether you tense up to the noise or are relaxed about it. The CBT was shown to work, by helping people to take a different attitude to it."
There has been little mainstream attention. Only a handful of articles have been published in scientific literature, including: Leventhall, 2004, 2003; Cowan, 2003; Mullins & Kelly, 1998, 1995; Broner, 1978; Vasudevan & Gordon, 1977. Others publications include: Fox, 1989; Wilson, 1979; Hanlon, 1973.
The Hum has been reported worldwide.
Although an obvious candidate, given the common description of the hum as sounding like a diesel engine, the majority of reported hums have not been traced to a specific mechanical source.
In the case of Kokomo, Indiana, a city with heavy industry, the origin of the hum was thought to have been traced to two sources. The first was a 36-hertz tone from a cooling tower at the local DaimlerChrysler casting plant and the second was a 10-hertz tone from an air compressor intake at the Haynes International plant. After those devices were corrected, however, reports of the hum persisted.
Two hums have been linked to mechanical sources. The West Seattle Hum was traced to a vacuum pump used by CalPortland to offload cargo from ships. After CalPortland replaced the silencers on the machine, reports of the hum ceased. Likewise, the Wellington Hum is thought to have been due to the diesel generator on a visiting ship. A 35 Hz hum in Windsor, Ontario, is likely to have originated from a steelworks on the industrial zone of Zug Island near Detroit.
One hum in Myrtle Beach, SC was suspected of originating at a Santee Cooper substation almost 2 miles away from the home of a couple who first reported it. The substation is home to the largest transformer in the state. One local couple sued the power company for the disruption the hum was causing them. The hum was louder inside their house than out, in part, they believed, because their house vibrated in resonance to the 60 Hz hum. The volume of the hum was measured at up to 64.1 db in the couple's home.
A suggested diagnosis of tinnitus, a self-reported disturbance of the auditory system, is used by some physicians in response to complaints about the Hum. Tinnitus is generated internally by the auditory and nervous systems, with no external stimulus. However, the theory that the Hum is actually tinnitus fails to explain why some hearers report that the Hum can be heard only at certain geographical locations.
While the Hum is hypothesized by some to be a form of low frequency tinnitus such as the venous hum, some report it is not internal, being worse inside their homes than outside. However, others insist that it is equally bad indoors and outdoors. Some people notice the Hum only at home, while others hear it everywhere they go. Some sufferers report that it is made worse by soundproofing (e.g., double glazing), which serves only to decrease other environmental noise, thus making the Hum more apparent.
Tinnitus is known to be exacerbated by allergens, which are regional and seasonal. Due to blockages, inflammation, and pressure inside the head, taking allergy medicine may help to mitigate the Hum.
Human ears generate their own noises, called spontaneous otoacoustic emissions (SOAE). Various studies have showed that 38–60% of adults with normal hearing have them, although the majority are unaware of these sounds. The people who do hear these sounds typically hear a faint buzzing or ringing, especially if they are otherwise in complete silence.
Researchers who looked at the Taos Hum considered otoacoustic emissions as a possibility.
One of the possible causes of the West Seattle Hum considered was that it was related to the midshipman fish, also known as a toadfish. A previous hum in Sausalito, California, also on the west coast of the United States, was determined to be the mating call of the male midshipman. However, in that case the hum was resonating through houseboat hulls and affecting the people living on those boats. In the West Seattle case, the University of Washington researcher determined that it would be impossible for any resonating hum, transmitted via tanker or boat hulls, to be transmitted very far inland; certainly not far enough to account for the reports.
The Scottish Association for Marine Science hypothesised that the nocturnal humming sound heard in Hythe, Hampshire in the UK could be produced by a similar "sonic" fish. The council believed this to be unlikely because such fish are not commonly found in inshore waters of the UK. As of February 2014, the source had not been located, although the sound has now been recorded.
A case of "hum" in a house, reported in the Daily Telegraph's 'Letters from Readers' on 18 January 2018, proved to be a wasps' nest in a hollow wall.