tfhavel

My subwoofer makes a continuous humming noise at what sounds like 60 Hz, even when the AVR it is connected to is switched off. The volume of the humming can be controlled with the "Level" dial on the back of the subwoofer, and the humming goes away when the RCA cable is disconnected from the back, so it seems that the cable itself is acting like an antenna and picking up ambient 60 Hz from its surroundings. Is there a way to shield and ground an audio cable like this?