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.