Hi, I've recently noticed a hum coming from my center channel speaker (CS100). The hum is heard only when the TV is on. When I play a DVD, the hum is gone. Could this be a broadcast issue with my cable company or do I have a real problem with the speaker? I bought all my speakers back in 1990 but I have a newer Sony receiver and a newer subwoofer. All the speakers are Polk and are in a basic surround sound config.