So far I have removed and inspected: Axles, diff (swapped out), driveshaft (incl: center support bearing/mount and flex couplers). I also put a new trans mount in, due to the one done by a dealer a few years ago had stripped bolt holes... All appears to be fine. It should be noted that the eng mounts were done apx 9k mi ago and the flex couplers and center support around 5k mi ago.

I just put the car on jacks and crawled under it while my coworker duplicated the noise for me. Both sides actual make a "pop"and shift a little when this happens (whole caliper assembly and suspension). Almost like a bushing in the suspension is bad. But since it happens on both sides, that doesn't seem right... I'm about to say screw it and drop it off at Mercedes to diagnose.