guys, can you describe the sound a bit. is it a One time sound or happens a few times when it does?
ALSO, bare with me, try this... vent or open your moonroof.
does the sound go away?
if so its a moonroof sound problem (very common on this board) that SOUNDS like its coming from the boot.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S