[Meotchh]

Sounds like it could be a loose heat shield on one of the catalitic convertors. I believe you have four on that car. I know there are at least two. That is just my guess without seeing the car or hearing the noise. It is also not uncommon for the heat shields to become loose & rattle as you are describing.

Pretty easy to determine or eliminate that possibility. Just get under the car & check to see if the shield(s) are loose. You could also tap to convertors with a rubber mallet to see if you hear the same noise. Do not touch any part of the exhaust system if the car is still warm from running, unless you have the proper heat protective gloves.