Granted, I'm new to driving a manual... But I haven't done anything terribly wrong (besides stalling the car a few times when i first started driving it) enough to mess anything up.

I've noticed that when I shift gears (specifically launching in first) there is a subtle rattle that lasts for a second or two coming from the back of my car. It sounds like it could be the exhaust almost, but I checked and the exhaust is bolted up rock solid so its not that. I had my dad take the car out (more experience stick driver than I) and I heard it when he was driving too.
What can it be? Anyone else have this problem (if you want to call it a problem)? Is it serious? I dunno if I'm just being paranoid cuz I'm so overly-conscious about me messing up the tranny on a brand new car or if its a real problem?

Any help would be appreciated.

Thanks