I installed my carpc and had a terrible hum coming from the speakers and back of the computer, around the sound card. After lots of searching and few answers I finally tried some stuff other suggested.
The first thing I tried- and the fix: Ground the frame (box) of the PC! I grounded my inverter to the seat frame bolt, so I just slipped another wire (16ga) under it and ran it to the most convenient place under a screw on the back of the PC.
Hope this helps someone.