No it's not the wiring as it only happens when switching from MP3 source to Tuner source using the remotes SRC button for source switching, never happened when switching w/o tuner. The thump or grumble describes it better, is only about a 1/2 second but if the volume is loud when switching sources the sound is really irritating. It's almost like the tuner is finding the frequency it was last on or the tuner is switching on.
I'm going to try it later using the menu switching to see if it does it then as well.

#695-Mk2/8 colors-12Gig/TUNER- awaits v2.x & Voice Recog.