Are you running amps off of that headunit above? If so, make sure to move the RCA's as far away from the unit as possible.

I had to do that or else I got alternator noise with the CB radio so close to my headunit.

You may also want to ground the headunit to the chassis instead of through the harness.

EDIT: I just read that you said it didnt change with RPM. I would still check the ground on the head unit and make sure its to a solid,clean, metal surface and definitely not through the factory harness for the radio...The same applies for the CB....

My ground wire was way below the radio as shown in my original pics.