A while ago I posted a thread about a faint, plasticy-sounding vibration/rattle that seemed to be coming from the rear seat area... after many helpful suggestions from this forum, I thought I tracked it down to the ski sack pass-through compartment. I tried various sound-proofing techniques to the area (felt pads stuck to all the areas that might vibrate, etc... ) but the rattle would still come back after several miles of driving. As a result, I eventually just removed the entire plastic ski sack assembly (which I'd never use anyway - skis are meant to go on a rack AFAIC) so now I have a rattle-free pass through area... but after another day of driving, the rattle returned again!!
Sometimes lowering the rear armrest would make the rattle stop, so I was still fairly certain that it was coming from the center of the rear seat... so after studying the back seat again, I thought maybe the headrests were the culprit... so I removed all 3 of them. But the rattle came back AGAIN!

I really have no idea what it might be at this point... My only remaining ideas are maybe one of the rear seatbelt assembies, or the interior panels of the C-pillars (they seem to make a little noise when I lightly tap on them with a fist) Other than that, I'm about ready to give up.
As you can see, this is
really buggin me, so if anyone has any further ideas, please help.
Or, if all X5's - even brand new ones - have minor rattles and buzzes of this sort, and there's nothing that can be done about, maybe it's time I just accept it (?)
Thanks,