Just bought my '08 g37 coupe (74k miles) about a month ago and everything was good until I heard a rattle sound coming from my exhaust. It rattles when I'm in park and when I go into 2nd gear at low speeds. I can also hear a rattle when I close my driver side door. I've been researching for a while and I wanted to diagnose the problem before I took it to the dealer. I'm still covered under my dealer's warranty so maybe it's just a heat shield and not something else.

 

There's a TSB for it just take it to the dealer and if you want look up the TSB number on Google. My 08 coupe used to the same thing never got to fix it tho

 

Did you check if there was any missing bolts. Last time when I got my muffler deleted, I experienced the same thing. It turned out that I lost a bolt close to where the muffler used to be.