Grounding problem?

[josh.pries]

I'm installing a sub, aftermarket speakers, a kenwood 4 channel amp, and kenwood head unit. Everything is hooked up and runs great. However I noticed that I'm getting engine noise through my speakers. This is usually a sign of grounding problems. So here's the weird part... There are 2 rcas running from my hu to the amp. One for the speakers, one for the sub. when 1 Rca is plugged in by itself there are some engine noises, when the other one is plugged in alone there is no engine noise, and when both are plugged in the engine noise triples. Do I have a bad rca that's running some sort of weird loop? Just very unsure of what to do here. The amp for the Record is very well grounded directly to the frame and the head unit is grounded with the wiring harness. I've tried touching the ground to the rcas with no results as well.
Thank you!