No, I did not try messing with the head unit ground, thank you for
suggesting it. What about the head unit ground would make the noise thru
RCA's but not hi-level inputs, even when the engine is off? A ground loop?
As I said in my first post, any info is appreciated.

Ground loops are funny sometimes. The addition of the RCA's could actually
cause the ground loop, your HU could be attempting to use those as a ground. Or
you could have bad RCAs with a break in the cable somewhere. Try the HU ground
and try a different set of cables. Pay attention to the results of each and
then in combination, that way if it doesn't work you can tell us what exactly
happened.

Les