This is actually very common. In many cases, the cable television shield is at a different potential than earth ground on the wall socket. It would be nice if they connected these together at the point of entry, but many times, they don't. So a ground loop results when you connect your cable to your television and to your sound system.

There's an easy solution if your television (and radio tuner, if connected) has 300 ohm antenna inputs. Get a 75-to-300 ohm transformer and connect the cable through it. This will isolate the cable shield from chassis ground and will break the ground loop and reduce the hum.

Wayne