Afaik, It's either interference or a grounding issue.
Make sure your amp is grounded properly, make sure it's metal contact (so you don't have the cable attached to paint. if it is on paint, file the paint down, and put the cable back)
As for interference: make sure your power cable and rca's are in different sides of the car... Doesn't always make it happen, but it can be a caused sometimes.
Power and ground together won't do much.. it'd be signal interference.

But yeah, check those things, come back and tell if it's still there. If so, check the headunit.