The Largs hum is a phenomenon that wreaks pain and misery on many. Now sufferer Georgie Hyslop has turned detective to try to solve this mystery. 

One morning, it got so bad Georgie Hyslop thought it was finally going to kill her: her chest constricted as if it had been tightly bound, then the noise intensified in her ears. At the same time, stabbing pains tore down her forehead and pressure began to build up in her nose. It got so bad she felt like her skull was vibrating in her head. She wanted to be sick, but was incapacitated by the pain. For an hour, she lay in bed in her Largs home expecting her life to end. ''I just wanted to die,'' she says.

It is a rare admission of despair from the quietly spoken 59-year-old who has spent the past two years living with the Largs hum. A low frequency whirring noise of unknown origin, the hum causes pressure to the head, nose, ears, and sternum, leading to headaches and chest pains, nausea and grinding misery.

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The low-frequency hum is a documented phenomenon all over the world and was first identified in Largs in the 1980s. But, in spite of its consequences, its cause is unknown, with possible explanations ranging from gas pumps to radar and transmission masts. Moreover, not everyone can hear it, even in the same street. As a result, it has tended to occupy the scientific twilight zone, somewhere between the Beast of Bodmin and UFOs.

But, in Scotland at least, that could be on course to change following the efforts of Georgie Hyslop. She last slept properly in her own bed more than 18 months ago. However, despite constant fatigue, she has pursued the source of the hum with the tenacity of Hercule Poirot.

Her dining room has become her incident room, adorned with maps and pressure-wave graphs, and strewn all over with lever arch files bulging with correspondence. Now, with her local MSP on her side and a Glasgow-based noise specialist preparing a research proposal with a view to seeking European Union funding, she is more confident than ever of finding an answer.

She has contacted more than 30 agencies, including six mobile phone operators, two government ministries, Transco, ScottishPower, the World Health Organisation, the European Environment Agency, Faslane naval base, Hunterston power plant, the British Geological Survey, and the National Radiological Protection Board. She has had a dozen engineers in her home: many have left with headaches and chest pains. She has driven miles establishing the position of more than 20 local transmission masts, as well as generators, pumps, and radar communications installations, and she has made contact with fellow sufferers all over the UK.

Her quest began with a telephone call to the council's environmental health department. An officer came out and took readings in the house to no avail. A statement issued later by the council read: ''At all times the measured levels were so low as to be below the threshold of normal human hearing. Furthermore, there was no occasion when any officer was able to hear the alleged noise. Accordingly, no statutory noise nuisance exists.''

They suggested she contact Scottish Power. An engineer arrived and the pair of them drove around Largs, visiting pylons where Georgie would report any changes in the sound's intensity. It did not appear to be related. The baffled engineer passed her on to Ingenko engineering. It also sent out engineers who reported that there was some sort of interference in the area, but were unable to say what.

Then came a breakthrough. Les Mair, a lecturer at Glasgow Caledonian university, came to the house with acoustic equipment. He detected low-frequency waves of between 50-60Hz, a frequency which falls within the spectrum of human hearing.

''I was pretty sceptical at first, but there does appear to be a problem which is causing severe distress,'' he said later. ''This situation is more prevalent than might be realised and is reported all over the world in very similar terms.''

Later, a Sky engineer, using a sophisticated spectrum analyser capable of detecting a range of frequencies, found an ultra-high frequency wave of 700-862 MHz. There could be more.

The findings, though far from conclusive, have led to the theory that the hum was caused not by one soundwave but two or more combining to create a ''beat frequency'' detectable by people such as Georgie with particular sensitivities in her ear.

But, even if the theory turns out to be correct, it does not explain why some people can hear it and others cannot. Middle-aged women who hear noises no-one else can are not all hearing the Largs hum. One local suggested that Georgie was hearing nothing more than a jetski on the Clyde.

But the sceptics have been outweighed by those who nod knowingly, recalling similar experiences themselves. More than 20 people in Largs have told Georgie they can hear the hum, although she stresses she has no way of verifying it. There is the man who has lived with it for years and calls her for support when it gets bad.

Then there is Jeanette Matheson, 58, from Paisley, a frequent visitor to the bungalow next door to Georgie who has been so harassed by the noise that she now ''dreads'' her visits to the town. And, of course, there are other sufferers from all over the UK, like Adele and Douglas Farquharson from Whitehills near Banff, who both started hearing the noise on August 5, 2000, in the house they had lived in for 23 years. It got so bad they have had to sleep in the car and take refuge in hotels.

But becoming a campaigner was the furthest thing from Georgie's mind when she moved to Largs following the death of her husband, Roy. His sudden death on July 1, 1999, following complications during a heart operation, came just seven months after the couple had retired to Wales. With no family nearby, Georgie, who was raised in West Kilbride, decided to move back to her native Scotland to be close to her two sisters and her brother.

On her first night alone in the house she became aware of her uninvited guest. ''I moved in on January 10 and by the eleventh, I thought, 'What have I done?','' she says.

After the noise and the headache came interference with an electronic implant in her back. Since then, nosebleeds, vomiting, and unyielding, inescapable noise have been her daily diet. She listens to the radio to drown it out during the day and sleeps in a tent in the garden at night when it becomes too much to bear.

Isla, her golden retriever and ''saving grace'', has shared in her misery, and the two often slip quietly down the driveway in the early hours to drive through the hills around Largs by moonlight, relieved to be free of the soundbox that her house has become.

But she has never seriously thought about moving and, instead, has taken on frowning officialdom. A low-level government report into the hum 10 years ago decided that it existed only in the heads of sufferers, as with tinnitus. Georgie, however, like countless others, has been medically examined and does not have the condition.

To overcome any scepticism, she has adopted a forensic approach: while other sufferers have jumped to conclusions about its source, 21 months into her investigation Georgie has ruled nothing out. A former radar operative for the RAF, with a tomboyish appearance and an unassuming manner, her grasp of the subject has served her well. ''For some reason no-one has put the phone down on me or failed to write back,'' she says. ''I've never found anyone who didn't agree to help or give me information.''

She is aided by experience. This is not her first campaign. While living in a Yorkshire village, a spate of accidents involving lorries headed for a local quarry prompted her to campaign for a new road. At a packed meeting in the town hall, she learned the importance of dispassionate, informed campaigning. ''Someone said to me, 'We only came here to try and talk you out of it','' she says. ''But after the meeting they said the reason they hadn't given me a hard time was that I'd been able to answer their questions. I've learned from that.'' She got her road.

She is also driven by a sense of justice. As well as losing her husband, Georgie lost her younger sister to cancer just four months after moving to Largs. When she found herself plagued also by the hum, she became angry.

She recalls her husband with tears welling in her eyes. ''If Roy were here, he'd be sitting laughing and saying, 'Not again'.

''He was a lovely character. He was only5ft 4in but he was 6ft tall as far as I was concerned. He never said a bad word about anyone. I certainly miss him a heck.

''When Roy was ill and we knew they had to operate, the doctor said, 'We do our job, we do our best, but we can't change destiny.' Within two hours I had lost him. But you either get up and run away or dig in and keep going.''

Other people, she insists, suffer more than she does. ''One woman who can hear the hum in Norwich is disabled and so is her husband. You get letters from people whose hearts are breaking but they still sit down and write, to me and to parliament. Some days you feel how can I go on? But you just jump in the car and go for a picnic. Not everyone is free to do that.''

And, in one way, it has been a positive experience, by bringing her a host of new friends. ''If it's a really bad day, you pick up the phone, hang the expense, and you know that you're not the only one suffering,'' she says. ''When I met Adele it was almost like we'd known each other for years.''

Following a visit to her house by Dr Bernardette McKell, a noise specialist from Stanger science and environment in Glasgow, accompanied by the parliamentary assistant to Kay Ullrich MSP, she feels confident that something will finally be done to establish beyond doubt the cause of her misery. Both could hear the hum and are preparing a pitch for funding.

And if, after all this, the source of the legendary Largs hum remains undiscovered? ''If at the end of the day we don't trace it, at least we have done what we can. But it's getting so bad and affecting so many people that something has to be done,'' she says. ''Personally, I'm more convinced than ever that we will get an answer.''