It sounds like a big tumble dryer, but it seems airborne rather than underground. That is the desperate question from New Waltham pensioner Margaret Belton (67), who claims the humming noises which have plagued her for three years are getting worse.

In May 2004, the Telegraph reported how unidentified humming and whizzing sounds were keeping Mrs Belton awake at her Pretymen Crescent home.

Since then a team of experts from the University of Salford have visited the location as part of a national research project into the "hum".

A fascinating report has proved the noises Mrs Belton hears are not just sounds in her head.

In a report by academics, an intermittent sound of between 40 and 60 decibels was recorded at the pensioner's home.

The findings were mirrored in other parts of the country where low frequency sounds were also registered in areas where residents complained of "humming" noises.

However, the cause of the noises remain a mystery.

Mrs Belton said: "It sounds like a big tumble dryer, but it seems airborne rather than underground. It has got so much worse - now I am waking up each morning at 5am and I know I am not the only one to hear them."

"I am not going mad - it's there."

A deaf friend visited the house and also detected the vibrations, while a puppy next-door but one is also affected by the whirring.

A summary of the report on the University's website reads: "Normal noise guidelines are not appropriate for low frequencies, so Environmental Health Officers have little guidance on whether to class a low frequency noise as a nuisance.

"Low frequency noise is also particularly difficult to measure reliably. This means that low frequency noise problems may go unresolved for years."

The latest theories

Low frequency sound is only detected by about five per cent of the population.

These are almost always aged 50 or over, with 70 per cent of them being female.

One of the latest theories is that the sounds are caused by dead people. Known as white noise, it is a pattern of sound which registers below most human beings' hearing range and is the spirit's attempt at communicating with the living.

Mrs Holton does not endorse this theory, but instead blames the noises on some kind of industrial equipment.

One of her most persuasive theories is that the noises are caused by electricity surges, made more plausible by her claims that the sounds worsen at 7am to 9am in the morning and 6pm to 8pm at night, which is when demand is highest.

Sounds like a good idea

The Department for Environment, Food and Rural Affairs (DEFRA) is now trying to develop guidelines for use by local authorities in cases of unidentified sounds. A study will focus on 10 cases where a cluster of residents is known to be disturbed by low frequency noise. The sound will be recorded over several days while residents keep a log of comments.

These 10 sounds will then be reproduced in a listening room and 16 people will be asked to comment on whether they find them disturbing.

They will then try various ways of rating the sounds according to their level, frequency content and, for example, whether they fluctuate or have particular characteristics.

The methods that give the best agreement with the reported disturbance from the field and lab trials will be proposed as a criteria for rating low frequency sounds