Just got a call from the dealer, so I thought it was worth updating this thread.

Evidently, the new and improved bottom bumpers/stops have rubber core and are filled with gel (as opposed to the old stops that were solid rubber). The ones they installed yesterday in my X5 have both exploded by the time I brought the car back in this morning, spitting the gel all over the tailgate (I didn't even know since I haven't used the tailgate since picking up the car yesterday). So much for the new and improved design. Fortunately, they dug up my old stops from trash and are putting them back in there, along with the new top bumpers/stops (hopefully those haven't been redesigned ). And there I thought that after today I would be rattle-free once again...