The always fascinating Things Magazine has a terrific entry about strange hums that have confounded authorities and hapless citizens for years.
More on hums, which some have dubbed the Taos Hum, 'a low-pitched sound heard in numerous places worldwide.'Link
So what is the Taos Hum? Spurred on by complaints that the 'Bristol Hum' (which has driven at least one person to suicide) was caused by faulty gas pipeline equipment, British Gas undertook an investigation, canvassing 33 hum sufferers. Of these, 80% were found to have hearing problems, but 20% were genuinely hearing something. Further investigation found that the noise was actually originating from a number of distant sources, including distant machinery, and were 'being amplified by the geometry of particular rooms' in the sufferers' houses.
Related, the concrete sound mirrors on the South Coast, designed to listen in for fleets of approaching enemy bombers in the days before radar.