ok this has happened like 3 times, and only recently.

i started it cold, and like right when i turned the key, and it started there was this horrible knocking sound coming from underneath the car like a huge metal peice fell on the ground or something.

the car ran fine after it happened....just wondering what the **** it was because it scared the **** of me.

 

Maybe the starter gear?

 

While its running, get under the hood and look for something obvious. Check all the moving parts, like the belts and pulleys.

 

it might be a loose part of your exhaust. when your engine vibrates, while trying to start, it might be shaking around and banging on your rear suspension cross member. Or maybe its an old troll in your storage compartment. Mine hates RX7s, so I feed him butterfingers and he shuts up.

 

you may be right about the exhaust banging around, ive heard it rattling a bit lately down by the muffler.