Tranmission noise?

[Anupgs]

Hi
I bought a new Auto transmission, 2004 premium package G35 Sedan. I have about 500miles on it. I just noticed a strange noise, when I start the engine and start driving the car and it shifts from 1st to 2nd gear, its makes a wired noise (like clunk). I have tested it several times, it only happens when I stop, restart the engine and then start driving it. It doesnt make any noise when it shifts from 1st to 2nd gear if my engine has been running.

Has anyone else noticed this problem?
Any ideas on what it could be?

Thanks
AG