Turns out the “rattle” in the Nismo (sounds like a marble going around inside of a washing machine) are parts that have broken off inside the catalytic converter. The good news here is it is covered by an 8 year 80,000 mile Federal warranty.
Talked to a friend who has a 370z and he had one replaced about 6 months ago. Not sure if this is happening to a lot of you but please do beware.
Cheers.