does your exhaust rattle when you are not in gear(6mt). I don't know if that is normal but mine does. When I try to bring to the dealership attention they couldn't find the problem.

 

Does it do it towards the rear of the car? If so remove the wedge piece from the front of the muffler & se if it goes away.

 

What I'm assuming is the exhaust on my 06 G35x makes a metal-on-metal pinging sound right below me when I'm not moving (it's probably there othertimes as well, I just can't hear it). I notice it when I go through a drive-thru or park my car with the window down. I'm going to have the service guys look at it when I go in for an oil change this week.