Who, What, Why: Why is 'the hum' such a mystery?

A village in Durham is the latest place to report a strange vibrating noise - known as "the hum". Why is it such a mystery?

According to sufferers, it is as if someone has parked next to your house and left the engine running. The Hum is a mystery low frequency noise, a phenomenon that has been reported across Britain, North America and Australia in the past four decades.

There is a range of theories from farm or factory machinery to conspiracy theories such as flying saucers. And yet, "the hum" remains an unsolved case.

Woodland, a village in county Durham, is the latest place to fall victim to the noise. Some residents have reported hearing a buzzing noise like electricity or a car engine that won't go away.

"It sounds like an overhead power line with this constant humming buzz," says Kevin Fail, a 53 year-old bathroom installer who lives in the village.

He said that he and his wife hear it in bed, downstairs in the house and outside in the garden, but some residents have heard nothing. Fail believes it may have something to do with a disused mine shaft in their garden.

Durham County Council says it is planning to send someone with sound monitoring equipment to the village to investigate.

There are "crackpot theories" doing the rounds about UFOs, and Fail says his daughter, whose hobby is ghost hunting, hasn't ruled out the possibility that the mine is haunted. But unlike some residents, Fail says he's not worried. "This has been happening all over the world for decades. Whatever's out there is not going to hurt you."

Another resident of the village said they had received media interest from all over the world.

"The hum" is an international phenomenon. The beach front neighbourhood of Bondi in Sydney was afflicted by it two years ago. One local resident told Australia's Sunday Telegraph at the time: "It sends people around here crazy, all you can do is put music on to block it out. Some people leave fans on.''

One case that was partially solved was in Kokomo, Indiana. The source of "the hum" was located to a fan and a compressor on an industrial site, and yet even after these were turned off some people complained the noise had not stopped.

The Largs Hum in Scotland and Bristol's mystery noise in the 1970s are two of Britain's most famous cases. Often the source of the noise is never found but disappears unexpectedly.

The truth is no-one really knows the cause of "the hum", says Geoff Leventhall, a noise and vibration consultant who has advised the government on the issue.

Despite years working in the field, he has never heard the hum himself and has only rarely been able to pick it up on recording equipment. In one case, his recording equipment picked up a 200 hertz signal at a complainant's house that was detectable in the lab. He managed to trace the buzz to a neighbour's central heating. But this, he says, was an exception.

"Some experts say if you can't measure a noise the presumption is tinnitus," he says. "It all gets rather fraught because people say there's nothing wrong with my hearing."

"The hum" is sometimes heard in cities but is more likely to be audible in the countryside and at night, when there is less background noise. Most complainants are people aged 50-60. The most plausible causes are industrial compressors and fans or farm machinery, Leventhall says.

In the 1970s he worked with the News of the World on their campaign to discover the mystery behind "the hum". They received 800 letters from readers complaining of the phenomenon - some of them citing UFOs. But no specific explanations emerged.

In 2009, Dr David Baguley, head of audiology at Addenbrooke's Hospital told the BBC that in about two thirds of cases no external noise could be found. He believed that sufferers' hearing had become over-sensitive. "It becomes a vicious cycle. 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."

In the end, the solution for sufferers may be to adopt a more accepting mindset, Leventhall argues. He prepared a report for the Department for Environment, Food and Rural Affairs that suggested cognitive behaviour therapy was effective in treating the symptoms. "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."

As for the source of "the hum", don't expect a breakthrough anytime soon, he says.

"It's been a mystery for 40 years so it may well remain one for a lot longer."