Computer scientists at New Zealand's Massey University are studying the mysterious low-frequency hums that have been reported in various places around the world. (It's sometimes referred to as the Taos Hum because many people in Taos, New Mexico claim to hear it.) Massey University professors Tom Moir and Fakhrul Alam began their research on "Unidentified Acoustic Phenomena" with a visit to the home of a Brown's Bay woman who said a hum she heard at home was sickening her. Moir and Alam couldn't detect the noise but are now expanding their efforts by talking with others who hear a hum. From a Massey University press release:
"The fact of the matter is that we do not yet have an answer even though there has been keen interest and plenty of speculation world wide on this phenomenon," says Dr Moir.
"At this stage we believe there are two possible explanations. The hum could be a very low frequency sound that only some people can hear. Or, it could be that microwaves in the atmosphere trigger a hum like sound in the heads of some people that would not necessarily be heard by others or picked up by recording equipment."
UPDATE: BB reader Gerard Hughes comments that it might be a little premature to speculate on a cause:
They left out the most important possibility–that there is no hum. Before speculating on what causes a phenomenon one should prove that it exists in the first place. There are a number of famous scientific self-deceptions when people thought they could see or hear things even though their instruments detected nothing. René Prosper Blondlot's "discovery" of N-Rays is one such example. He thought the rays made a calcium sulfide thread glow very faintly but it turned out that the researchers were just fooling themselves. Link