had the same prob i has a 2000 4 dr blazer all it is road rash or interference make sure ur remote and rcs are ran on the other side from the wires for ur amp and make sure ur ground is really grounded good if not ull have it forever trust me its annoying as **** good luck

 

Also make sure your HU is grounded very good!

 

X2 on the headunit ground. You can get the thump because of a bad ground there. If all else fails, ground the hu on bare metal or to the hu case itself.(it's worked for me before) On mine the sound of the laser moving and the disk spinning is coming thru, with the engine off. No music at all, and I bet mines the hu ground also. Good luck and keep us updated.