I have only heard it on 856, 'Audio Visions', the OP states it's on XM channels in general, I believe him, I just changed to it now, while typing this, and I can hear the popping every couple seconds still, the pops seem to pretty much go away when there is (louder) music playing, until a silent part, than you hear it again, it may occur on other channels too, but I have not yet noticed it on the XM channels I frequent (very few), that may or may not be because of the type of programming on 856 (quiet music) allows easier detection of the pops.