If you listen closely enough, you might hear it.

For years, people all over the world have reported hearing a strange and persistent humming sound. Not everybody can hear the humming but those who say they hear it describe it as sounding something like a distant idling diesel engine. Usually, the low-frequency noise is hard to pick up with microphones and no one can figure out exactly where it comes from.

Many people who claim to hear the humming sound say they can hear it more clearly inside buildings instead of outdoors. Some say they can also feel vibrations throughout their bodies. The sound is often heard more intensely at night and earplugs are little help. Those poor folks who can't sleep during the night because of the sound are often called "hum sufferers," according to a Web site about the weird phenomenon.

The humming has been reported all across the globe but it has been especially prevalent in Europe and New Zealand. But during the 1990s, the humming phenomenon first became known in North America when a study by the University of New Mexico and complaints from people living near the city of Taos, N.M. were widely reported by the national news media. Here in the United States, the phenomenon came to be known as the "Taos Hum."

The popular show Unsolved Mysteries devoted a segment to the Taos Hum. The show, then hosted by Robert Stack, even contained a recreation on audio tape of what it sounded like. That was because the audio equipment for the show was unable to adequately pick up the low-frequency sound, according to a Web site.

In November 2006, a professor with the University of Massey in Auckland, New Zealand reportedly made a recording of what is called the Auckland Hum and has posted it on the university's Web site. And just this year, Dr. David Baguley, the head of audiology at Addenbrooke's Hospital in Cambridge, England, said he managed to successfully record what he believes to be the humming sound.

There are various explanations for what causes this weird noise. Some say the source is man-made, such as stereo systems or distant industrial sounds. Others suggest the sound is actually the noise from electromagnetic waves caused by meteors when they enter the Earth's atmosphere. Another explanation could be that extremely low-frequency communication systems are causing the sound. Of course, some believe that the humming noise is being made by alien visitors from other planets.

And some hold that the noise is actually being generated from within those who claim to hear it. The proponents of this theory believe that the hum sufferers need to seek out medical treatment, not advice from paranormal investigators. However, the theory that the humming is actually tinnitus fails to explain why it can only be heard at certain geographical locations.

So if you listen closely, you might hear the humming.