make sure the rca cables are not crossing the power/ground wire down by the amp. are you running your speaker wire parallel with the power wire? this could cause a problem. but i would say make sure that you are not crossing rca's over power/ground in the trunk. try and route the rca around the amp, avoiding all power wires, and then try. also, if the gain is cranked up high, it will be more susceptible to funky sounds.