Mains hum
Mains hum is a signal at a nominal 50 or 60 Hz, depending on local alternating current frequency, and usually some harmonic content. The changing magnetic flux caused by the mains current links with the windings of the pickup, inducing a voltage by transformer action.
Mains Hum Media
50 Hz hum of an AC power system
Hum of an 60Hz electrical power system
Hum of an 400Hz electrical power system