Your engine noise problem is due to a ground loop. Its entirely possible that its a problem inside the head unit itself, or could be a bad ground at the amps, speakers, anywhere. Do you have power wires running alongside your RCA's? Because the RCA's are supposed to ground to the head unit chassis inside the head unit, and its possible they aren't. For a detailed explanation on grounding and engine noise, go here:
http://www.eatel.net/~amptech/elecdisc/caraudio.htm
Look up the heading "vehicule chassis ground". This website is, by far, the best and most informative web site I have ever encountered on car audio, and highly recommend it to all.
Let me know if it helps...
