Kr4 is right, but add this: if your cable box is attached to your HT setup, a poorly/non grounded cable coming in to the house could cause ground loop hum. Very Common. At least try removing the cable box from the system.
HELP: how to eliminate speaker HUM?
I've developed a nasty, fairly loud hum from all five speakers in my HT setup as well as my sub. If I turn off amp, all humming stops. If I disconnect IC from preamp, that channel/spkr hum stops. Is this ground loop hum? (I've heard of this but don't know what it means). I've heard there may be devices that stop the hum ("humbusters"?). Please help. Thanks..Jeff