The dealer added the strut for me at no charge. For some reason BMW decided to eliminate it. The cargo door will not stay open without it and I suspect that getting your tire out should you need to, would be tricky.

I tried the tape where the doors latch to the vehicle and to each other, and it seemed to help, but there was still a similar rattle noise from somewhere else - that strut assembly I suspect.

I may just get the dealer to take over at this point.