When his electrical engineering job was transferred to Peterborough, Hugh Witherington leapt at the chance to get out of London. But only months after moving out of the capital to a cottage in the Hertfordshire countryside, Mr Witherington, then 33, was woken in the small hours of the morning by a low, droning hum. 'It sounded like a lorry had stopped outside my window and left its engine idling,' he explains. He got up to investigate, but found nothing close to the house.

Unable to sleep, he drove through the surrounding country, searching for the cause of the noise. That was 16 years ago. Since then, Mr Witherington has been plagued almost continuously by the intrusive hum, and is still trying to establish its source. 'No amount of exposure ever gets me used to it. It stops me from thinking, even now. I cannot accept it,' he says.

The Hum, a persistent, low-frequency noise, audible only to certain people and with no identifiable source, was first reported in the mid-1960s. It has plagued people across the country and indeed the world. Some doctors have dismissed it as tinnitus, but the latest research, carried out at Addenbrooke's Hospital in Cambridge, suggests that the noise is, as hearers have always insisted, external. The Hum is most apparent in the countryside, at night, and is amplified by certain buildings, driving acute sufferers to sleep outdoors. It has generated endless speculation, and a conspiracy theory or two, but remains a scientific enigma.

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Sufferers, known as 'hummers', have blamed defence radar systems, microwave levels, the increase in noise pollution and, most frequently, British Gas high-pressure transmission pipes. Some have even attributed it to rock faults or residual noise left over from the Big Bang. All agree on its effect. The Hum shadows its victims like a vast, invisible bluebottle, causing sleep loss and chronic stress, disrupting health, relationships and careers and, in a few cases, inducing personality changes and driving people to suicide.

'I can only describe it as a kind of torture,' says 46-year-old Rosemarie Mann, of Romsey, near Southampton, who became aware of the noise in 1987, and has since wondered whether her life is worth living.

Mrs Mann, whose hearing has been checked and found faultless, is acutely sensitised to The Hum, which she compares to a massive industrial installation throbbing in the distance. 'For the first few years I lost sleep, couldn't concentrate and was unable to do anything. I was constantly in tears, which put a great strain on my husband. It has changed me from an active, creative person to a stifled, angry pessimist,' she says.

Now on anti-depressants, she, like many sufferers, finds the only way to escape The Hum is by masking it with the hissing noise made by an electric fan. 'The Hum still has a terrible draining effect. It is the invasiveness and relentlessness of the noise and having no understanding or control of it which makes it so distressing.'

Like many hummers, she finds the condition isolating. 'Most people can't hear the noise, so when you talk about it, they assume you're 'hearing things' and think perhaps you're going mad,' she says. 'We get tired of the ridicule and disbelief and end up keeping quiet about it, but we are, I think, a vital early warning system for greater difficulties to come.'

Over the past three decades, reports of The Hum have become more frequent and widespread. Hugh Witherington, who founded the Low Frequency Noise Sufferers' Association in 1989, claims that local authorities receive around 1,000 complaints about low-frequency noise each year. The LFNSA has 600 members, but Mr Witherington estimates that at least 10,000 people in Britain now hear The Hum.

In the last few years, he has been contacted by 'hummers' from Australia, New Zealand, Canada and the United States. In 1991, The Hum suddenly appeared in Taos, New Mexico, where it now affects hundreds of residents and has driven scores to move away. Mr Witherington, an advanced skier, has travelled all over the world for his sport and heard it in all but a few locations. He fears that The Hum will soon pervade the entire globe and inflict misery on a sizeable group of people.

'Rubbish,' says Dr Jonathan Hazell, head of research at the Royal National Institute for the Deaf. 'Everybody who has tinnitus complains at first of environmental noise. 'Hummers' are a group of people who cannot accept that they have tinnitus. About 4 per cent of them may genuinely be hearing low-frequency noise, but this shouldn't be a problem. Low-frequency noise has always been with us. It's caused by the wind and the rain.'

Dr Hazell believes that the hummers' problem is psychological. Sound perception is linked to a primitive survival reflex, he explains. Our hearing amplifies sounds that we perceive as life- threatening, such as the hiss of a snake, or a baby's distress cry, and filters out those we interpret as innocuous. 'The question to ask 'hummers' is what they think the sound will do to them,' he suggests. 'They perceive it as harmful and threatening, so their brain's response is to amplify it.'

'These people will sometimes not hear the noise when they're away from home and you'll often find that they're living in a home environment that's gone badly wrong. They scan their environment for the cause of their distress and latch on to this noise.' The cure, says Dr Hazell, is for the sufferer to reinterpret the noise as non-harmful and so reduce their awareness of it. And the Taos Hum? 'It's like appearances at the Virgin Mary isn't it? It's very much a question of belief.'

Needless to say, Dr Hazell's comments go down like a lead balloon with the LFNSA. 'Doctors back a theory and they have to stick with it, whatever new evidence comes to light,' says Hugh Witherington.

A former RAF radar systems engineer, he is convinced that he has located The Hum's cause. On first becoming a hummer, Mr Witherington spent nine months driving around Hertfordshire at night with his twin brother and fellow sufferer, Martin, who lives in the same hamlet. After eliminating suspects such as the ventilator fan of a nearby battery farm, they narrowed it down to a high- pressure British Gas pipeline a few miles from their home. 'That was my very last suspect,' says Mr Witherington, who believes the spread of The Hum ties in with the installation of the high-pressure gas transmission network.

British Gas denies these allegations and points out that The Hum also coincides with the spread of motorways and trunk roads.

After 15 years of trying to get to the bottom of The Hum, Mr Witherington is now working on the theory that it has been the subject of a government cover-up. Why, he and fellow sufferers ask, has no serious government funding ever been put into researching a problem that destroys the lives of hundreds of people?

In 1992, the Department of the Environment awarded a two-year contract, worth pounds 50,000, to a little- known private laboratory to research The Hum. Part of the Sound Research Laboratories' brief was to develop new instruments to measure low-frequency sound, a task that would, sufferers argue, demand far larger funds. The project's research findings, will be ready in September.

Hugh Witherington complains that the research has not substantially involved LFNSA members. However, David Baguley, of Addenbrooke's Hospital, who headed the medical research side of the project, says his findings will 'raise a few eyebrows'. Unfortunately, he cannot disclose whose. At this stage, all he will say is that low-frequency noise is the 'dustbin' of the sound spectrum. High frequencies attenuate and disappear more quickly, while low frequencies travel further and penetrate matter more easily. British Gas pipelines are just one of many generators of low-frequency noise.

It remains to be seen whether the 'hummers' are a valuable group of sound pollution 'canaries', or tinnitus sufferers in a state of denial. Den Nash, a self-employed computer software designer, moved with his family to an idyllic part of Dorset in 1982. Shortly after the move he became aware of The Hum and has been plagued by it ever since. 'It has disrupted my whole life for 12 years,' he says. Fortunately, the noise made by his computer partly disguises The Hum. 'My work has helped me escape from it. But to sleep I have to put on a fan or radio, which keeps my wife awake. We can't share a room any more.'

He hopes that one day soon a billionaire will become afflicted: 'That's the only way it's going to be researched properly.' In the meantime, he's looking forward to moving back to London, where the general din masks The Hum. 'That's the only place I can get any peace and quiet.'

For further information send pounds 2.50 to Elisabeth Griggs, Secretary LFNSA, 6 Hyatt Place, Shepton Mallet, Somerset 5A4 5XY.

(Photograph omitted)