Question about cold start up

[Omegamerc]

Back when I had my 05 Altima - when it was a totally cold start (ie its been sitting for 6hrs or overnight), while starting the car (ie turning the key and waiting for the engine to give the first "vrmm") I would ALWAYS hear some kind of metal vibration or metal hitting metal noise which would always last the same amount of time and happen at the same point and only last about 1-2 seconds.

At best I can associate the noise to the exhaust line vibrating around (I know this from my work van).

Now third week into owning my 07 G35 and AT THE SAME EXACT POINT the SAME noise happens and lasts the SAME EXACT TIME! It's driving me nuts! It only happened once but the noise was the same harmonic tune/duration. As soon as the first "vrrrrmm" windles down it happens, anyone know whats up?