Finally solved the ground loop on the boat, discovered the solution by chance.
I had to tie the outside (ground) of the RCA audio cables running to the amps to the chassis of the PC.
Things bounced around once and noise disappeared, wiggled things around and noise came back. Looked around found out cables had bounced around enough to touch the chassis.
TruckPC - gutted, being used for test setup
BoatPC - All in 1 in cuddy, N7 using VNC on dash, RR
BlazerPC - Nexus 7 with Timur's USBROM