Swap your speakers wires...switch your front speakers to the rear amp and then rear speakers to the front amp. See if the problem stay with the one amp or it follows the speakers.
If it does it on the fronts then it is a problem with that one amp. If it follows the speakers maybe one of your speaker wires is pinched and touching ground somewhere.
Just start doing process of elimination in this case.
Makes sure your amps are grounded well. (no paint) Make sure your computer is grounded well.