I'm noticing this as well. Some channels seem worse than others. Heck, some songs seem worse than others. I was starting to think it's an encoding issue on XM's side... I notice that the comedy and talk channels are particularly egregious, but also The Villiage (15), Bluegrass Junction (14), and some tracks on Air Musique (100) have noticeable artifacts. If I listen particularly to cymbols or when the singer/speaker hits a fricative consonant like "s" or "sh" or "f", I can hear white noise above the vocal track.
I think I'm going to find someone with a different receiver and sit my car nex