I had a similar problem like this at one point. You might want to consider unplugging your RCAs, turning your head unit on and plugging them back in. It is possible that your head unit or amplifier is trying to ground through them as well. if not, I would think that it is some kind of electrical interferance from a nearby power wire. Double check to see that they are as far as possible from each other at all points. (I know with mine they come near each other close to my sub amp.)

Hopefully this helps.