Why is my subwoofer making a humming noise?

Whenever we are done watching a movie and the movie ends, we hear a humming noise from the subwoofer. It is all plugged into a Monster Power surge and line conditioner, as is all the av components. If I turn down the gain on the sub, it goes away, but there is no b during the movie. Turn the gain up, everything seems okay until like I said, the movies over. It is an older JVC 5.1 receiver, and a JBL 10" sub.Sub is powered and only has line from receiver and power to Monster strip. Any ideas?