There are a few things that can cause this. Very dirty power lines in the house can easily pick up radio station signals. If you have very very long speaker leads, you can end up picking up small amount of RF interferrence (or if your speaker cables run close to, or parallel with AC power lines). I know nothing about your system, but inexpensive audio equipment can cause bleed across inputs. If the interference happens on all inputs, i'd guess it was dirty power, or the amplifier bleeding across inputs. If this isn't the root of your problem, try moving your speaker cables around if possible. Finally, if you don't suspect your amplifier is causing an issue, pick up a power line filter.