Hi SylBer,

It's almost certainly a ground-loop hum, and you'll hear it in all the speakers plus the subwoofer. Here's my guide to eliminating the hum with links to devices that will help you get rid of it:

http://www.axiomaudio.com/tips_eliminate_hum.html

Disconnect the cable-TV feed (or satellite-TV feed) from the cable box or satellite box and see if the hum goes away. It does not matter whether the box is turned on or off.

Regards,
Alan

Alan Lofft,
Axiom Resident Expert (Retired)