Some residents of Auckland, New Zealand, have been complaining about a mysterious and uninterrupted hum haunting the country's largest city.

The low-frequency noise is audible only to a small number of people. But for some, it is so bothersome that they have put their homes up for sale or have started taking anti-anxiety medication.

Tom Moir, professor at Massey University's Institute of Information and Mathematical Sciences in Auckland, recently succeeded in recording what's now being called the "Auckland hum."

Although he hasn't been able to identify the sound, which he can't hear, Moir doesn't believe theories that attribute low-frequency hums to alternating current flowing through electricity grids.