Help: Exhaust rattle

First time poster, long time reader. I have a 2001 330ci which has a bizarre rattle. Whenever the car is under load or facing down hill, if the RPM's are between 1500-2000 there is a high pitched rattle somewhere under the car ( I think). The sound is like a coin in an aluminum can being shaken. On a prior car, I had this sound and it was the heat shield. Well, no heat shield on the BMW.

I have taken it to the dealer and two repair shops - no luck in isolating the sound. The sound ends once the RPM are above the 2K mark. Oddly, if I down shift and RPM's are falling, the sound comes back between 1500-2000 as well.

I did run over a tire prior to the sound beginning, and the central muffler has a small dent in it. Still, the sound could not be reproduced on the lift when taken to the repair shops. I am tempted to just replace the central muffler, but that is around 1K per the dealer.

One shop told me the sound is because there is a valve in the exhaust which opens at certain RPM's and is the cause. Also, they said the sound is normal. I don't remember it being there before I ran over the tire though.

Any thoughts?