It is very hard for any shop to warranty against noise. It is quite often product related. Sometimes a simple ground loop isolator removes the noise but at what cost? If it removes the noise, it is also removing sound that lay at the same frequency. So in essence, it is a band aid at most. I would try the following things. Grab a cd player instead of the REQ5, wire it in and listen to it. Use the same power and ground as the REQ5. If you still have noise, keep looking, if you do not, chances are that the REQ5 is the culprit.