Strange one mate - had a similar issue with a carputer system recently though, couldn't find the source and even earthing the chassis of the system to the car seperately didn't solve it (it reduced the noise but it was still there). In the end I used a Ground Loop Isolator from Maplins for £8.99 - completely solved the problem.

I guess you don't want to try that because it will remove some of the lower frequencies of the sounds?

Cheers,

Andy