Noise, but only when using multiple RCAs

I've got a bit of an odd problem.  I'm getting engine noise when hookuping up more than 1 set of RCA's to my HU.   I have tested each cable individually and everything seems fine with no noise but as soon as I add a second set noise is a problem.   The headunit has 3 RCA out's (Front/Rear/Sub) and there is no noise on the first RCA regardless of which HU RCA out I'm using.    Any ideas what might be causing this?  

For refrence - my amp is grounded directly to the battery (in the truck in my Ion anyhow, I figured I might as well ground directly since it's right there.   I measured for both reistance and voltage between battery and HU ground and all seems ok there as well.   

Since each RCA works indepentandly of the others with no problems, I'm guessing this may be a problem with my HU but I'm looking for any other thoughts before I do a HU swap.

Thanks.

James 

without any other info, I might suggest taping your rca connections at the deck as they can sometimes touch each other inside the dash...  

Sounds like the deck then, because the rear outs and the front and sub outs are on seperate sides of the deck, so shouldn't be an issue there.   It's a Pioneer DEH550MP deck.  Frustrating, but I'll try pulling the Alpine from my wife's van and see if that clears it up.   She only needs one set of outputs in her van anyhow :)

James

Nope, never posted anything about this before.   Also, I tried using my wifes deck this afternoon and had the same problem - I even tried making sure there was no interferance by running new RCA's directly back without putting them under the carpet just to be sure there was no onboard power source causing the problem.  I guess it's looking more and more like a fault in the amp; Either that or I have the worlds worst luck with two fault head units.   The odd thing being, I'd think it was something in the amp the noise would increase with volume, but it doesn't so at the moment I'm stumped as to where else to check.

Another update -  the noise goes away if I turn the gains down to around 25% on the amp.  Unfortunatly I don't seem to be getting nearly the SPL expected at those levels however.   No clipped or distortion problems at higher gains, just engine noise.

James

I'm running 4 gauge power and ground directly to battery.   All connections are solid and reconnected.  I have tried both twisted and untwisted RCA's, both giving me the same issue.   As I said, it's bizzare.

James