Originally Posted by REV2 8K
What you could try is running whats called a trace ground. Where you run a new ground wire (16 gauge would be fine) from your deck all the way back to your trunk and ground it with the ground for your 4-ch amp. If there is even a 1 volt difference in grounds, you will get engine noise. Make sure the ground for your amp is screwed directly to body metal (ie. trunk floor or floor pan under rear seat, etc.) with the paint scraped away.
I have two amps and an EQ (with inline power filters on the power wires) all running off the factory head unit and I have no engine noise.
I was gonna suggest that but forgot about this thread. lol A trace ground will remove any ground noise just like you said. I should have done mine that way, im starting to get a buzz in my system.