The Hum

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.[10]

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.[14] After those devices were corrected, however, reports of the hum persisted.[19]

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.[20] Likewise, the Wellington Hum is thought to have been due to the diesel generator on a visiting ship.[21][22] A 35 Hz hum in Windsor, Ontario, is likely to have originated from a steelworks on the industrial zone of Zug Island near Detroit.[23]

One hum in Myrtle Beach, South Carolina 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.[24] 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.[25]

Some sources claim that very low frequency radio waves or extremely low frequency radio waves that are used to communicate with submarines might be the source for the hum.[26]

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.[27] Tinnitus is generated internally by the auditory and nervous systems, with no external stimulus.[28]

While the Hum is hypothesized by some to be a form of low frequency tinnitus[5] 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.[29]

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 shown that 38–60% of adults with normal hearing have them, although the majority are unaware of these sounds.[30] The people who do hear these sounds typically hear a faint hissing (cicada insect like sound), buzzing or ringing, especially if they are otherwise in complete silence.[31]

Researchers who looked at the Taos Hum considered otoacoustic emissions as a possibility.[32]

One of the many possible causes of the West Seattle Hum considered was that it was related to the midshipman fish, also known as a toadfish.[33] 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.[34] 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.[35]

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.[36] The council believed this to be unlikely because such fish are not commonly found in inshore waters of the UK.[37] As of February 2014, the source had not been located, although the sound has now been recorded.[38]

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.