update: picked my x up this afternoon and the problem seems to be resolved. i drove over the same bumps on my way home that consistently cause the knock/rattle from the rear quarter and i couldn't repeat the problem. i want to drive it for a few days to be sure but hopefully the problem is solved.

here's what the dealer did, quoted directly from my service receipt:

loud knocking noise from the rear
caused by 2nd row seat strikers are out of adjustment
adjust the 2nd row seat strikers for both sides

also, when i dropped it off i complained about the steering wheel peeling and they replaced the trim piece under warranty.

hope this helps.
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Facts are stubborn things --John Adams