Finally got around to dropping off the car for the rattle noise. They checked the exhaust and said everything was tight. It might just be the intial start up and quites after 30 seconds. Normal I guess.

definetly sounds like your cat or heatshield is rubbing against something when you turn it on. does the sounds get worse when you downshift?

Mine does this as well, and like the others goes away when it is warmed up. Always figured it was normal.

It sounds normal through the down shifts. They wanted me to leave the car overnite but I passed on that option at this point. Others here mention they hear a noise at an initial cold start up. I was comparing it to my wifes 2010 Maxima that does not have this brief sound after a cold start up.

I mentioned that to the dealership and they naturally said the two cars had completely different exhaust. I was like mehh crap. Oh well, I most likely will pay them for an oil change and leave it overnite. Something tells me this brief start up noise is going to end up being the nature of the beast. :ugh2:

Mine never did it with stock exhaust, but since i went Still CBE and Berk HFC's at every cold start it does it.

rattling noise at start up
 

I had the same problem on my 2012 370z. Nissan's responce was this: "I am very sure the noise your hearing is a normal cold start situation produced by the variable valve timming. The noise is caused due to the valve timming being advanced or retarded allowing what they call valve overlap.This allows the internal compression in the cylinders to be reduced for easier startups and for quicker warmups.As the engine runs the timming is returned to normal idle settings lessening the noise.

Typically the noise produced is a tinny sound though the tubular header type exaust manifold.If the manifolds were cast steel the performance and sound would be greatly reduced because of the exaust flow and sound dampening ability vs. steel tube header type manifold"

It isn't normal. I actually got my muffler replaced by Raceway Nissan here in riverside for free. It is a baffle that is in the exhaust that is loose. It isn't normal. I have a youtube video of it from standing out side the car

Rattling exhaust 370z - YouTube

My bone-stock 2009 Touring has done that since day one. Local dealer checked it out and said it wasn't an exhaust leak. The dealer I use for service now has told me "they all do that" and said it has something to do with emissions control during startup (pretty much the same answer gq4life1345 got from Nissan).
My guess is that what you are hearing is normal, but, being it's the exhaust system, I'd get someone who is qualified to look at it - if for nothing else than peace of mind. Until then, stay alert for symptoms of carbon monoxide poisoning.