Pontiac Repair: Rattle at Startup, dull thud, spray paint

Question
Hi Todd,
I have 2001 Firebird, V6 3800.  When the engine is cold, after I start it up, there is a rattle for about one minute.  It sounds to me like and exhaust/muffler rattle.  It doesn't happen when the engine is warm.  I thought it might be a loose catalytic converter heat shield, but I checked it, and it appears to be securely fastened.  Any ideas.

Answer
Hey Mike:

It's either a heat shield on the exhaust pipe or manifold or the catalist is loose inside the converter housing. That has kind of a dull thud/rattle noise. if it's hight pitched or like a spray paint can shaking up it's a shiled on one of the pipes. Good luck :}