Right i'm tearing my hair out on this one, I'm hoping somebody can help.
I'm fitting an aftermarket Parrot headunit in place of the standard one, and I have a non-Bose system in my S3.
I have installed using the correct adaptors taking low level signal to the amp etc and it all works, apart from an annoying high pitched hum in the rear speakers without playing music or even switching the head unit on.
Now i've fitted a ground loop isolater and tried all sorts of stuff, and still does it even if I remove the head unit, all of the Autoleads adaptors back to the standard Audi connectors and then just bridge the power from the brown plug straight to the large black amp plug it still does it, but if I connect all the plugs back in to the Audi Concert head unit it stops....
I think i've effectively eliminated all of the gear i'm putting in from the cause of it, so thats one thing.
I really can't figure this out unless it's something to do with the Audi head unit stepping the current/voltage down or something??