At least one suicide in the UK has been linked with the hum
For decades, hundreds of people worldwide have been plagued by an elusive buzzing noise known as "the Hum". Some have blamed gas pipes or power lines, others think their ears are faulty. A few even think sinister forces could be at work.
"It's a kind of torture, sometimes you just want to scream," exclaims retired head teacher Katie Jacques.
Sitting in the living room of her home in the suburbs of Leeds, the 69-year-old grandmother describes the dull drone she says is making her life a misery.
Most visitors hear nothing, but to Katie the noise is painful, vivid and constant.
"It has a rhythm to it - it goes up and down. It sounds almost like a diesel car idling in the distance and you want to go and ask somebody to switch the engine off - and you can't."
Katie says she no longer has any quiet moments and getting a good night's sleep has become impossible.
"It's worst at night. It's hard to get off to sleep because I hear this throbbing sound in the background and you know what it's like when you can't get to sleep and you're tossing and turning and you get more and more agitated about it."
Katie first became aware of the maddening rumble two years ago. She turned everything electrical off at the mains, but that made no difference. Neither did her efforts to block out the sound with ear plugs, or smother it with music.
Katie Jacques: 'It's kind of torture'
Neighbours are unaffected and tests by environmental health officials have drawn a blank.
Checks on Katie's ears ruled out tinnitus, a ringing noise that generally follows the sufferer wherever they go.
Katie, like most victims of the hum, only hears the noise at a specific location - in her case, at home. Elsewhere, her hearing is fine.
Moving out is an option she's considered, but she's reluctant to leave the house she's lived in for nearly 50 years.
"My children grew up here, they still live nearby, so do my grandchildren. I have lots of friends here. I don't want to move, but I have thought I may have to if I can't find out what's causing it."
The hum is a phenomenon that has been reported in towns and cities across the world from Vancouver in Canada to Auckland in New Zealand.
In Britain, the most famous example was the so-called "Bristol hum" that made headlines in the late 1970s. One newspaper asked readers in the city: "Have you heard the Hum?" Almost 800 people said they had.
The problem persisted for years. Residents complained of sleep loss, headaches, sickness and nosebleeds. Experts eventually found traffic and factories were to blame.
There have been other cases in Cheshire, Cornwall, Gloucestershire, London, Shropshire, Suffolk and Wiltshire.
A low-pitched drone known as the "Largs hum" has troubled the coastal town of Largs in Strathclyde for more than two decades.
People assume you must be hearing things, but I'm not crackers
Katie Jacques, hum sufferer
At least one suicide in the UK has been linked with the hum.
And the problem is on the increase, according to the Low Frequency Noise Sufferers' Association. Two thousand people have so far contacted its helpline, and it says it receives two or three new cases every week. They are generally over 50 and are mostly female.
So what is the cause? Various features of modern life have been blamed - gas pipes, power lines, mobile phone masts, wind farms, nuclear waste, even low-frequency submarine communications.
The internet is abuzz with rumour and speculation. There are dark mutterings about secret military activity, alien contact and government cover-ups. The hum even featured in an episode of the sci-fi drama "The X-Files".
Such conspiracy theories are understandable, but unhelpful, according to Dr David Baguley, who's head of audiology at Addenbrooke's Hospital in Cambridge.
He estimates that in about a third of cases there is some environmental source that can be tracked down and dealt with.
"It may be a fridge or an industrial fan or a piece of heavy machinery at a nearby factory that is causing the disturbance and can be switched off," he says.
Most of the time, however, there is no external noise that can be recorded or identified.
"People do come up with some strongly constructed, sometimes strange theories," says Dr Baguley.
The more people focus on the noise, the more anxious and fearful they get
Dr David Baguley, audiologist
His own theory - based on years of research - is that many sufferers' hearing has become over-sensitive.
Surrounded in his office by plastic models of human ears, he explains how we each have an internal volume control that helps us amplify quiet sounds in times of threat, danger or intense concentration.
"If you're sitting by a table waiting for exam results and the phone rings you jump out of your skin. Waiting for a teenager to come home from a party - the key in the door sounds really loud. Your internal gain is sensitised."
This is a mechanism we all rely on at moments of pressure or stress when we want our senses on full alert.
According to Dr Baguley, the problem comes when an individual fixes on a possibly innocuous background sound, and this act of concentration then triggers the body's "internal gain", boosting the volume.
The initial "signal" may vary from person to person, but the outcome is the same.
"It becomes a vicious cycle," he explains. "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."
Sound of silence
In an attempt to break this cycle, Dr Baguley is currently working on a pilot project with the acoustics laboratory at the University of Salford.
The trial - funded by the Department for Environment and the Department of Health - uses psychology and relaxation techniques to help sufferers become less agitated and distressed by the hum.
Dr David Baguley has examined numerous people with the problem
The experiment is not finished, but Dr Baguley says the initial results look promising, allowing the noise to quieten and in some cases fall silent.
"It's really exciting," he says. "For years I've been seeing people with this problem in my clinic and it's been hard to find answers. But now there is hope and there is potentially help."
Back in Leeds, Katie Jacques is pleased the hum is being taken seriously, but remains adamant that her suffering is caused by a real, external noise nuisance.
She suspects it may be something to do with the nearby airport, although the authorities there say no engines are left running overnight.
"People assume you must be hearing things, but I'm not crackers," she laughs.
"I don't know how I can get this over to people, but this is not in my head. It's just as though there's something in your house and you want to switch if off and you can't. It's there all the time."
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