More than likely the noise is entering through the DC power input.
If you can, try running the XM receiver with its internal batteries to see if the noise goes away. Then get a good DC Noise Filter and splice it into the power cord. Try Ham Radio Outlet (about $30).

Noise can also enter on the audio cable between the XM Reciever and the Car Stereo. To block that try running the audio cable through a ferrite core (3 or more loops through the core if possible).