Not a stupid question, I had the same problem and saw Confused grounded his case and all was well.
However I checked my case and the whole thing was already grounded, and the resistance to ground was as low as it could be for the ground wiring - I ended up using a ground loop isolator and it sorted everything in seconds.
Ford Focus MP3 : www.stevieg.org/carpc Blog Updated 29 January 2009! Car PC Status: Complete - Undergoing Software Redevelopment
On my first system, with an inverter & ac psu, when I first turned the system on on the car, I had a REALLY loud buzzing on my speakers, and the PSU was under the passenger seat while the computer was under the dash and the amps were in the trunk! I found out that the ground pin on the inverter ac outlet was connected to nothing. When I connected it to the car chassis (effectively grounding the psu case) the buzzing went away completely. That means to me: always ground the psu. However, it won't help to cure alternator whining, that one is a lot more complicated...
Im getting interference from my PSU out of my speakers, is this because my PSU case is not grounded?
is that a stupid question?
I just came inside from repairing mine!
Yes, I grounded the psu straight to a good ground.
Also, run your sound wires as short as possible and as far away from the power wires as possible.
Mine came out great, completely quiet. (Other than music)