new stereo noise

Doing some garage boating hoping for warmer weather I noticed a low static like noise in the inboat and tower speakers. Been trying everything I know and read the related articles on here but still can't figure it out. The HU runs the 4 inboat speakers, and single amp runs 4 tower speakers and a sub. All the HU wires are in a harness(done at a shop), I tried bypassing the harness and running the power and ground to the amp and still have noise. Gain is set to about half on the amp. Am I missing something? If I disconnect the rca input to the amp it stops the tower speaker noise, thats why I was concentrating on the HU and ground. Any thoughts?

Okay, so via the process of elimination you have determined that the cause is not internal with the amplifier. Continue to use the same logic to isolate each upstream component in the signal path.
If you tried wiring direct then I assume you have the noise without the key engaged?
Do you have the noise with all sources, ie; CD, radio, ipod/phone/aux?
If it is exclusive to radio then you could have a disturbance in the garage such as lighting.
If it is on all sources then you have to separate the HU from RCAs.
Make sure that all RCA cable ends are well seated with a fair grip. Try gently moving around the RCA cables at both ends. Use a substitute RCA cable.
Besides the HU harness ground, try grounding the HU chassis.
No meaningful diagnosis should take place without first verifying that you have a healthy voltage.
Also, check the voltage across the amplifier primary terminals with the amplifier under load.
Any action that creates a change in status should automatically narrow your focus.

Just tried different RCA's, no deal. I do need to replace the battery this year, didn't even think of that. I guess I'll change the battery before I get to carried away. thanks

Trace all your grounds all the way through the boat. I had a similar problem last year in my Tige, turned out to be a loose ground on the primary fuse block. Tightened up all the connections and all is well now.

all tight. I did add a 3rd 12v acc plug that I had to double up on the ground bar. maybe I'll pull it off to make sure that's not the issue. still haven't replaced the battery though. Its not to loud and realistically I wont hear it when outside, but now I know its there so its going to bug me. thanks for the advice.