Lets go back to square on. What components do you have make and model of radio and xm tuner and where are they located in the car? The first thing I would recommend is to unplug the sony xm unit from your radio and determine if you have noise still. If you don't then it would be fair to say it is coming through the sony's audio cable and there is more than likely a ground loop being created between the two components or the sony unit is picking up noise on its power line which can be filtered with an inline filter. You can also try grounding the shield on the rcas going into the auv input, to the radio chassis and see what difference that makes. Let me know what you find !

Scott@lajollaaudio

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The Mercedes Specialists Since 1986