I thought it was the spare tire, so I tightened everything down and that did not take care of it. Then I thought it was the plastic handle that is used to hoist the tire out of the well, or perhaps another plastic part rattling around down there so I secured everything I could find to no avail.
The whole rear gate seems to rattle with every bump...it sounds like it hasn't been closed securely even though I know it has. This is on my list of issues for the dealer when I drop the X5 off tonight.
I hope it can be fixed, but from what I have been able to read on the forums it seems to be somewhat normal. Given BMWs attention to every sound their cars make, I am really suprised this by this one.