Rewinded? Is that a word?
This is a typical issue most people face when either 1) The grounding wire isn't grounded properly and 2) If the RCA's run along the same side as the power cable to the amp. Unless you are using shielded RCA's, this will happen.
without playing any music, turn the volume up and start driving the car. See if the shoon shoon sound increase as the car accelerates.