Originally Posted by KuttinEdge
I doesnt sound like a grounding problem. Usually when something isnt grounded properly, you dont get noise at all and if you do its like a popping noise not a buzzing noise. Now I am only assuming that the buzzing noise is coming through the speakers and not from the amp itself. If this is the case, try replacing the RCA cables with ones that are shielded. The buzzing noise may be caused by the AC frequency of the alternator.
something that is not properly grounded can cause a buzzing noise like that. its its not grounded right, noise can be introduced to the system really easily.