I had similar problem once with my 300E I had a seperate amp connected to the head unit by a shielded RCA wire for signal.I used the unit for a long time and all of the sudden started to hear engine noises in the speakers according to the rpm.I called the shop who installed the unit at the start and they told me to get a little better RCA cable preferrably gold plated plugs ,better shielding etc.I did that and that was the end of my problems.