It started quite suddenly, an unwelcome intrusion into an island’s natural soundscape. And unlike the ebb and flow of ocean crashing on rock or the storms that soak the lochans and bogs of Lewis, there has been no let-up from this disconcertingly unnatural interloper. To some, it sounds like a lorry’s engine idling in the road outside their home. Others complain of a infuriatingly vague but persistent low rumble they cannot escape. Most agree it’s worse at night when they’re praying for sleep to free them from their purgatory. Yet when they pull open their curtains, there is nothing out there to explain the incessant droning slowly eroding their sanity. Since it first assailed their ears in February, sufferers from Ness in the North to Scalpay in the South-East have tried in vain to locate the source of their despair. So far, all they have come up with is a name for their tormentor – the Hebridean Hum. To the vast majority of the island’s residents who can’t hear it, that may sound like an overly jolly name for a phenomenon which has blighted the lives of the minority who can. Disrupted sleep is only the start of their unpleasant symptoms: sufferers report difficulty concentrating, headaches, nausea, dizziness, and ‘fluttering’ in their eardrums. In extreme cases, it can engender feelings of isolation and deep distress. ‘It’s impossible to ignore – like somebody shouting in your face constantly for attention,’ according to Dr Lauren-Grace Kirtley, who has set up a Facebook page to support the dozens of so-called Hummers, who are being driven half-mad by it. Some say they are the only ones in their families afflicted by its constant thrum, while one mother in the coastal hamlet of Shawbost reported being at her wit’s end after her baby ‘stopped sleeping at night’ and her son complained of nausea and headaches. ‘It is a very low humming, droning, pulsating noise. It’s incredibly intrusive and distressing,’ Dr Kirtley, a doctor and university lecturer, told BBC Radio’s Good Morning Scotland programme last week. ‘I haven’t slept a night through for weeks and have problems concentrating. I get a lot of fluttering in my ears. It’s making me dizzy and giving me headaches.’ Some islanders say they are so distressed by the noise, they are considering leaving. One posted: ‘Awake at 12am, 2am. Sat outside at 4am with a coffee. ‘Been awake since. I can’t keep going with no sleep, ears constantly pulsing and ringing. Definitely going to relocate back to the mainland God willing, this is too much.’ Dr Kirtley, 44, who moved to her dream home in Aignish from Staffordshire two years ago, has teamed up with fellow sufferer Marcus Hazel-McGowan, 52, a physics teacher and amateur radio enthusiast, to find the source of the Hum. M R Hazel-McGowan, who moved his family to the island partly in search of peace and quiet, is a former regional manager for the Radio Society of Great Britain and used to ‘chase up spurious emissions and sounds’. He has begun mapping locations where the noise is detected and has found it to be less noticeable in the centre of the island and strongest on the east coast, telling the BBC: ‘It’s just trying to narrow it down and hoping nobody loses their mind completely over it.’ Using a machine called a spectrograph, Mr Hazel-McGowan has measured the Hum at 50 Hertz, which falls below the hearing range of most. Only 2-4 per cent of the population are thought to be able to pick it up. Dr Kirtley said the noise was known to be man-made and was not simply a background hum from appliances. She said: ‘It’s a persistent, environmental tone that can be heard indoors and outdoors in multiple areas.’ Following complaints to environmental health officers, the local authority has pledged to carry out further investigations. However, a council spokesman cautioned: ‘Due to the geographical separation of the reports, Comhairle nan Eilean Siar is currently considering them unrelated.’ While Dr Kirtley presses on, compiling evidence to link the cases, her Facebook page has become a lightning rod for a slew of theories attempting to fathom the cause. From the first report by a householder in Ness who wondered if a recently-installed smart meter might be to blame, contributors have run the gamut from the plausible – power lines, power stations and phone masts – to the downright bizarre, including interference from Russian subs or even the mating calls of amorous fish. It is also clear that Lewis’s Hum is creating an international buzz, with interest from as far afield as Canada, the US, France, and Australia. And it’s not just armchair conspiracists offering their tuppence worth about sinister interference by government, the military, or, of course, aliens – the Facebook group has also been contacted by eminent names in the field of low-frequency noise pollution offering the benefit of their wisdom. And these experts are as one in their belief that, as incredible as it might seem, the Hebridean Hum is part of a World Wide Hum. Indeed, Lewis is far from the first place to endure low-frequency noise disruption. Last year, council officials informed the residents of Immingham, in Lincolnshire, that the source of a mysterious humming noise that has plagued them for years may never be discovered. Also last year, in a rare victory, a mystery hum dogging people in Omagh, Northern Ireland, was tracked by environmental health officers to an unnamed business premises and dealt with. In 2013, marine scientists argued that an outbreak in Hythe, near Southampton, might have been caused by ‘the mating call of male midshipman fish’, nocturnal creatures which ‘emit ever-louder drones, sometimes for hours’ to warn off other males. Midshipman were previously found to be the culprit when houseboat residents in Sausalito Harbour, California, reported strange noises. The truth is that ‘The Hum’ has largely baffled researchers since the 1970s, when the first widespread reports of the unexplained acoustic phenomenon cropped up around the West Country city of Bristol. In 1977, two of them wrote in the scientific journal Applied Acoustics that low-frequency sound waves generated by distant industrial sources were their best guess. Equally inconclusive was another famous hum which began plaguing the Ayrshire coastal resort of Largs in the late 1980s. The Largs Hum was the same low-pitched drone, inaudible to most but debilitating to a sensitive few. In New Mexico, the phenomenon is known as the Taos Hum after an artists’ enclave where the US’s first large-scale incident occurred in the early 1990s. A Congressional investigation was ordered, but experts failed to find any conclusive results. In Canada, it is known as the Windsor Hum following a spate of cases in the eponymous town in Ontario. Hopes that the Lewis Hum might be caused by the temporary switch-on of electricity company SSEN’s Battery Point Power Station in Stornoway were dashed when it was realised that the energy hub only operated at certain times, while the hum has been reported as constant. In any case, localised industrial sources seem inadequate to explain the worldwide prevalence of the Hum. So what exactly is this mystifying noise? And does it even have an environmental origin, or is it all in the mind? It is fertile ground for research not only for scientists, but science fiction writers too. Last year, BBC One addressed the issue in its four-part drama The Listeners starring Rebecca Hall as a teacher pushed to the brink by a low hum that no one around her seems to pick up. In Drive, a 1998 episode of The X-Files, a man is being driven mad by a painful sensation of pressure building in his head. Agent Scully, played by Gillian Anderson, discovers the cause is a United States Navy antenna array emitting extremely low frequency waves. Certainly, long-distance radio transmissions have been put forward as a possible cause of the Hum. One intriguing possibility is so-called TACAMO aircraft –military planes that employ radio frequencies in the lowest end of the spectrum to track or communicate with submerged submarines. T HE planes often operate at night, and their movements are top secret. Hum hearers in Largs have long suspected it is connected to operations at the nearby Faslane naval base, although no proof has ever been presented. If TACAMO was to blame, it might also explain why many sites are on the coast. The Russian Navy has long operated in the deep Atlantic waters off the Western Isles. However, the theory holds little water with the world’s foremost Hum scientist, Dr Glen MacPherson, of the University of British Columbia, in Canada. Dr MacPherson, a Hum hearer who set up the World Hum Map and Database Project to record instances, said: ‘I rejected that theory years ago after physical experiments ruled it out.’ So, what does Dr MacPherson, who has undertaken years of research, think it could be? His study appears to show that fully ambidextrous people, and those with a family or personal history of ADHD or autism are over-represented in the data. He said: ‘We are reasonably certain that the Hum is an internally generated perception of sound – that is, it is not actually a sound, just as tinnitus is not actually a sound. It is likely caused by some combination of specific anatomy, environmental exposure, or prescription/over-the-counter drug use.’ It seems astonishing to contemplate that all these reports of debilitating symptoms might simply be people’s brains playing tricks on them. Back in 2009, Dr David Baguley, who was a leading audiologist at Addenbrooke’s Hospital in Cambridge, told BBC News the Hum might be due to our sense of hearing becoming greatly heightened during times of stress. He suggested hearing about the Hum could lead sufferers to fixate on a perceived background noise, with it becoming a source of increasing frustration, causing additional stress which tricks the brain into turning up the volume even further. With his own Hum patients, Dr Baguley said he had some success with simple relaxation techniques borrowed from psychology. However, noise and vibration expert Geoff Leventhall, who has studied similar incidents for more than half a century, received short shrift when he advised the Lewis Hummers to try cognitive behavioural therapy to help them ‘relax and desensitise themselves’. ‘It is draining, debilitating and incredibly distressing and disruptive,’ Dr Kirtley told one newspaper. ‘Telling people to get used to it is not an acceptable solution.’ For that, they must wait and hope as the source remains, maddeningly, just out of reach.