Rattle from Exhaust

[littlex5]

My 2005 4.4 X5 is making some wild rattle noise during hard acceleration. It appears to come from the rear of the car, possibly the exhaust. Sometimes the noise is louder than other. It's pretty random. Guess what, when I took it to the dealer, the noise was very faint and the mechanic couldn't hear it. So I made a video and hope someone here can tell me whether this is normal or something wrong. It gets to a point I dread accelerating onto the freeway.

One thing I should point out is that the video was made with the microphone mounted outside the car taped onto the rear bumper. I did that to better record the noise. The noise is very noticeable inside the car with all the windows closed. The mic just doesn't pick up the noise very well when placed inside.

http://www.youtube.com/watch?v=wdYfio0xVkU

Thanks!