clunking sound at startup

ok, in my '98 GA 4door GT, when i start my car, it starts just fine, but when i let go of the key after starting it, it sounds like there is a bolt bouncing around in bouncing around inside some where in my engine, it only does it for a second, then every thing runs fine. What would cause this?

the enging is a 3100, w/ auto tranny w/ 68,600 miles on it.

tomorrow i think im gunna have some 1 start my car for me and try and figure out atleast the approximate area of where it's coming from.