The AV switcher you bought is maybe an inexpensive unit and may not design to eliminate interference properly. It's hard to find one that does better job in less than $100. The problem is with the ground.
If you use anything else other than composite then get a Receiver/Amplifier for $250 and it's a money well spent.
One thing is to try turning the power of the unit not being used and start unplugging each unit to find out which unit causing the interference.