It's not feedback, it's hum.
Two possible causes: unscreened/defective audio cable OR lack of mains earth or duplicated mains earth.
To explain the latter: in an audio chain -- for example a CD player connected to an amplifier -- the CD's power cable will not include an earth wire but the audio wiring's neg (screen) is connected to the amplifier casing via the input sockets. The amplifier's power cable must include an earth wire.
That's the principle, I hope it helps.