No one else in Phil Ciofalo's neighborhood in northeast Albuquerque by the foothills of the Sandia Mountains is bothered by the humming sound that irritates him constantly. They can't even hear it.

In other neighborhoods around the globe, however, Mr. Ciofalo has company, other people who complain of hearing a persistent humming sound, usually when they are in their homes seeking peace and quiet from a busy world.

''These people are definitely not crazy,'' said Jim Cowan, senior consultant for Acentech Inc. in Cambridge, Mass. Acentech was hired by the City of Kokomo, Ind., to study a mysterious hum that residents first complained about in 1999. ''They are just picking something up that others can't,'' Mr. Cowan said.

The preliminary investigation in Kokomo has determined one possible source for the hum, but like other studies it concluded that there could be several causes and that more research was needed.