Although 56Hz is within the standard range of human hearing - which can range from 20 to 20,000Hz - it is too low for most people to pick up. That however, has not brought the sleuths any closer to pinpointing the source of the hum which they have dubbed the Unidentified Acoustic Phenomena.

Dr Moir rules out geological factors. "It's more likely to be things like pipes under the ground - you know, gas pipes, sewerage pipes, factories in the distance." But for those who can hear it, the sound is the bane of their lives, driving some to distraction and others to take drastic action. Dr Moir said one sufferer, a man, was so desperate to stop hearing the sound that he deliberately tried to damage his own hearing by cranking up a chain saw close to his ears. "He said it was so bad, he couldn't stand it. It was driving him mad."

Another victim of the hum says it can prevent her from sleeping at night. Since a woman living in the North Shore suburb of Brown's Bay first contacted Dr Moir and his colleague Dr Fakhrul Alam in mid-August, the scientists have been approached by about 30 sufferers, all from areas in Auckland's north.

"These people who pick [the hum] up have a very low threshold for hearing at low frequencies - don't know why, but they do," says Dr Moir. Some have been reticent to give away more details of their predicament for fear that reports of persistent humming could adversely affect the resale price of their homes. With the help of one of the sufferers, Dr Moir has developed a simulation of the sound. "The real thing," he says "is more like the drone of an aircraft and it comes and goes," he said.

The affliction appears to be similar to tinnitus, a condition in which sufferers hearing a constant, high-pitched ringing sound. In severe case it can affect sleep and normal daily routines. A number of high profile musicians are said to have suffered from tinnitus including Beethoven, Bono and Eric Clapton.

Complicating the investigation is the fact that neither Dr Moir nor his colleague can hear the sound so with each sufferer they visit they must first run tests to rule out psychosomatic factors and any other obvious causes. Fortunately, Dr Moir discovered on the weekend that his wife, who accompanied him on a visit to one of the affected homes, was able to hear the hum. This is not the first incidence of humming in New Zealand. In 2005, New Zealand author Rachel McAlpine wrote a book called The Humming.

In her novel set in small town, an artist called Ivan and a number of the townsfolk are plagued by a low frequency humming noise. The book was largely inspired by the author's own experiences in the seaside town of Puponga on the northwest tip of New Zealand's south island which was itself at the centre of a humming mystery some years back.

Listen to a simulation of the sound (courtesy of Phil Strong). Most people won't be able to hear it, but if you play it in something like Windows Media Player and turn on the visualisation you will "see" the sound waves.