These so-called 'hummers' had hoped to hear that their protracted search was over. Many had become convinced that the source was the compressor stations at the end of British Gas transmission pipes.
But at a meeting in Solihull, near Birmingham, yesterday, three of the company's scientists dismissed the theory.
There are a few thousand known hummers in Britain - 500 new cases are reported each year. The hum, described by one sufferer as the sound of a car engine running constantly a few hundred yards away, is a minor irritant for some, but others have had their lives devastated. Some are on tranquillisers and cannot sleep. One has reportedly been driven to suicide.
Suggestions for the cause of the hum abound. They include air- conditioning, underground water pumps, factory noises, road vehicles and electro-magnetic fields around radio transmitters. But representatives of the Low Frequency Noise Sufferers Association had hoped the answer lay in the gas pipes.
'Our analysis was well received,' Pat Weatherilt, acting head of environment at British Gas, said. 'These people have a deep conviction about the cause of their suffering and the association with our pipelines, which is not based . . . on logic or science.' In a report published yesterday, British Gas analysed all the complaints the company received over the past decade. It concludes: 'The amount of low frequency sound energy emanating from the national transmission system is negligible compared to that from other sources, for example, motorways and trunk roads.'
It goes on to say that British Gas compressor stations operate, on average, for only 16 per cent of the time. 'They cannot therefore be the source of the continuous noise of which most sufferers complain.'
In several individual cases the scientists identified the source of the hum, and if this was a British Gas site it remedied the problem. But the researchers point out that low frequency sounds are less readily absorbed by air or objects in their path as high frequency noises. Potential sources could therefore be 'many and varied, and often impossible to identify'.
The scientists concede that sufferers are 'sincere indidviduals' who are in many cases caused considerable distress. They suggest that age may play a part - older people are general accepted as being less sensitive to high frequencies.
Earlier this year the hummers had their suffering acknowledged. The Department of the Environment is funding a two-year study in which scientists are trying to find the source of the hum for 25 sufferers.
Representatives from Ofgas, the industry's watchdog, attended yesterday's meeting. A consultant is liaising with the Buildings Research Establishment which is co-ordinating the Government's study.