For at least the past 50 years people have reported a disturbing low-frequency hum. Is it a man-made noise, possibly the cumulative effect of our technological activity? Or a natural sound generated by the Earth? Or maybe not an external noise at all, but generated somehow within the body?
The first reliable reports of the Hum came from Bristol in the mid-1970s when about 800 people reported hearing a steady "thrumming" sound, followed some years later by similar reports from Largs in Scotland. Then US reports emerged from Taos, Kokomo and Hueytown amongst others. Reports from all over the have since been documented in The World Hum Map and Database Project , established by Dr Glen MacPherson, working in Vancouver.
It’s thought that between two and four percent of the population report this phenomenon, and what many “Hummers” (i.e. those who say they can hear the sound) find particularly distressing is that most of those around them can’t hear this hum at all. Research by Dr MacPherson shows that the deaf and the hard of hearing are also amongst those who report the hum.
For most people the Hum is just above the threshold of hearing and descriptions of it vary significantly. This is partly because we do not necessarily have the vocabulary to clearly define a very low frequency noise, and these vague descriptions of the sound hampers research into the phenomenon.
Although its exact description tends to vary from person to person and from location to location, the most common characteristic is that of a low, throbbing hum similar to that of a lorry parked some distance away. However, a few Hummers describe it as sometimes being felt more than heard.
Some Hummers describe it as always audible wherever they are, for some it goes when they change location. Others describe it as audible only at night or indoors. One Hummer described it as “a kind of torture, sometimes you just want to scream”.
It is often reported that the phenomenon has only emerged in the past few decades, but this is not the case. Before industry became widespread, those who heard the Hum sometimes likened it to the noise of a swarm of bees – an apt description from a period when there were no rumbling engines or industrial fans to blame.
Early modern reports were concentrated in English-speaking countries where the Hum was recounted in the local media, but as the internet developed and spread the phenomenon became more widely recognised and it soon became obvious this was being heard across the world.
Human hearing responds to pressure waves in air from a low pitch of about 20 Hz (20 cycles per second) to a high pitch of about 20,000 Hz - although most adults can't hear beyond about 16,000 Hz and this frequency falls with age. The described frequency of the Hum varies, but it's generally assumed to be below about 100 Hz. To put this into perspective, the low hum heard from say a battery charger is 50 Hz or 100 Hz. Middle C is 256 Hz and the time pips for the hour on Radio Four are 1000 Hz. Infrasound is sound below about 20 Hz and cannot normally be heard, but if strong enough may have other effects. For example, it can be felt as a vibration, and is sometimes blamed for the feeling of deep discomfort or even fear in some haunted locations.
Ultrasound is above 20,000 Hz (20 kHz) and although it cannot be heard by most humans, dogs can hear up to 50 kHz, and bats use ultrasound for their echo location as high as 200 kHz. Those with a musical background may find the musical instrument/frequency chart helpful to describe the audible sound spectrum. (Click on the image to expand it for clarity.)
Suggested causes of the Hum are diverse. According to Geoff Leventhall - a senior acoustician who specialises in low-frequency sounds - possible urban culprits do in fact include acoustic noise-makers: “Ventilation fans on large buildings, air compressors, diesel engines, either as traction sources like buses, or stationary, such as diesel generators, air or water pumps".
Here are two typical examples the sources of which have been found; in Windsor, Ontario the hum stopped after a nearby steel mill was closed. In Bristol, two Oxford scientists eventually identified the source of the Bristol Hum as a large industrial fan at the Shell facility in Avonmouth. When it was decommissioned the sound disappeared. However, in recent years reports of the Bristol Hum having returned has reignited local debate as to the cause .
One definite identifiable cause is that of electrical equipment which often generates a hum of 50 or 100 Hz (60 or 120 Hz in the Americas). I helped a relative track down the source of an irritating hum only he could hear. It turned out to be an electric toothbrush charger in a cupboard three rooms away!
High-pressure natural gas pipelines criss-cross the country and these may be another source of the Hum for some people living nearby. A once secret network of pipelines established in the early 1940s, is used to deliver aviation fuel to military air bases and civilian airports. The type of earth and rock underneath all these pipelines may amplify the noise for people with sensitive low-frequency hearing, or may resonate the fabric of a building at the frequency of the vibrations.
One interesting theory was proposed by French oceanographer Fabrice Ardhuin, whose research suggested that waves coming from the seafloor generating micro-seismic activity, causing the Earth "vibrate like a bell," creating the Hum. Although other researchers consider this sound to be far too weak to be heard.  Despite the phenomenon being around for centuries, radio signals have also been suggested as a cause, and it was during early microwave radar experiments when some development engineers reported “hearing” the buzzing of the radar pulses if they got dangerously close to the radar antenna. But that’s caused by rapid heating and expansion and contraction of water in the head and is known as the microwave auditory effect. However, there is no evidence that EMF frequencies other than pulsed microwaves can cause any effect that can be interpreted as sound. 
One cause of the Hum often proposed is that of tinnitus – ringing in the ears – which is a medical condition. Some Hum researchers point out that this is unlikely to be relevant because sufferers would be equally distributed across the country (see later). Other researchers also reject it as the cause describing tinnitus is a high pitched sound, which the Hum obviously isn’t. But as a sufferer of tinnitus myself, I know that the condition can be a low-frequency rumble. Mine is exactly as many have described (with a measured frequency of 72 Hz), and sounds just like that of a lorry’s engine idling some distance away. When I first noticed it I walked around the house and garden looking for the source!
An audiologist and ENT consultant both confirmed to me that tinnitus can indeed be perceived as a low rumble, a whistle, buzzing, swooshing or even musical. So the condition might certainly be the cause for some people, particularly those who can hear it wherever they go, even when down in deep mines in an attempt to escape the noise.
This theory has led to scepticism as to whether the hum actually exists as a physical sound. In 2009, the head of audiology at Addenbrooke's Hospital in Cambridge, Dr David Baguley, said he believed people's problems with the Hum were based on real sounds in 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. This is known as hyperacusis which is a hypersensitivity to ordinary sounds. "It becomes a vicious cycle” says Dr Baguley. “The more people focus on the noise, the more anxious and fearful they get, the more the body responds by amplifying the sound, and that causes even more upset and distress”. 
Dr MacPherson who collated data for the World Hum map said in 2019: “It must be stressed time and again that there are many sounds created by human activity that can sound like the Hum, and it takes some effort and knowledge to track those sounds down. They range from electrical noise, pumps, industrial machinery, and so on. Once we eliminate those sources, we are left with the worldwide phenomenon that I am studying.” Research by Dr MacPherson (a Hummer himself) also shows Hummers get relief for a couple of days following a three or four hour flight which he says is significant, suggesting a therapeutic effect which prevents tinnitus for a while. In a recent video he strongly suggests many of those who hear the Hum - including himself - have tinnitus and suggests more research would pin this down once and for all. 
Here are a few suggestions I found circulating - which I have not included - as possible sources of the Hum: motorways, micro-seismic activity, wind through power lines or over mountain ranges, high-altitude winds, wind turbines, 5G, secret military operations, a mind-control experiment, some sort of alien intervention, even a type of a type of noisy mating fish.
It seems to me after reading some of the research into my own condition that there are probably two main explanations. The first being that the phenomenon is a real and measurable sound, often only detectable by certain people. A sound usually generated by some sort of man-made activity. This can be inferred from the clusters of reports around a particular location.
The second is that it's not a delusion, neither does it have some sinister source; it is probably tinnitus. These individuals are equally spread across the country. Researchers such as Dr MacPherson say that even when a number of Hummers are gathered together in one place, they tend to describe a different sound from which they are unable to escape, suggesting they are not all hearing a specific source. 
As technology and industrialisation spreads, so does its associated noise. As the internet connects people, we can share our experiences and draw conclusions, which are not necessarily accurate. I think both these facts lie behind the apparent rise in reports of the Hum. Like so many of the mysteries we enjoy, unravelling the facts and tracking down the evidence sometimes reveals the less interesting possibilities…
 Microwave auditory effect - Wikipedia