i get a rattling noise too on my 04. always think its a loose exhaust bolt or something.. but mine only happens at low speeds..

 

I went to the dealer for my second oil change and finally got an answer on this noise: the exhaust resonator.

(it's considered normal)

 

could your exhaust be banging against something?

 

From what i gathered by asking lots of people, it is that when there is a sudden drop in rpm the change in the speed of the exhaust gases going through the pipes means changes in pressure, kinda of like creating a vacuum. As this happens air wants to go back into the exhaust in fill up the void, it seems that the easiest way for air to get back in is through the flex joint that is why i'm getting the sound from there. It's a guess. I Put a wet towel over the joint and the sound was gone/ muffled.