I don't know if this is the same thing, but I've noticed that my '05 X5 4.4 has always had an unusual sound from the exhaust under full throttle acceleration (around 1500 miles now). I wouldn't call it a rattling...more like a staccato "fluttering" sound...weird.

I swear, the first time I heard it, it reminded me of the sound you got when you attached playing cards to your bicycle to make them flap on the spokes!!