Let me start off and say that I did search, and my problem is odd in that it is the reverse of any that I could find when searching.
I got rid of all of my speaker noise for the most part. HOWEVER, I get alternator whine when listening to the headunit with the carputer off. If I turn the carputer on, silence. Not a hint of noise, and my amp is turned up.
Specs are as follows:
dell laptop, powered through invertor and ac power supply
sony x-plod craptastic deck.
all monster cable speaker wires/rca's
run to a kenwood 600w amp which powers the speakers.
All audio equipment is run to a common ground (although the deck ground is a bit long... I took it off the chassis and ran it all the way to the back with thick wire, not sure of AWG, to ground in a common spot with everything else in an attempt to get rid of the noise. Didn't make a difference. )
The noise is only with the carputer off. It doesnt matter whether or not the headunit is on, as long as it is plugged in it makes the noise.
The signals are mixed by a simple audio mixer (shielded) which I made on a radioshack board.
I don't think it is my mixer because if the carputer is on, there is absolutely no noise at all.
If I disconnect the RCA's from the deck, the noise also dissapears.