WTF, what size muffler do you have? fair enough a mouse.... but a cat?
I had a similar sound in my corolla (yeah i know) and i found out that it was the guards between the exhaust and the bottom of the body of the car, its designed to stop the heat going into the cabin, have a look at that and tighten up all those bolts under there
HTH
EDIT: just re-read this thread, defs have a look at the cat for the smell, may be causing the rattle too.
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