Hey I get this rattle at start-up under the car; seems like it is coming from just below the drivers seat. I am thinking maybe it is the heat shields clanging, because it doesn't make any noise after that.

Any ideas other than heat shield that can cause this? Leak in the exhaust system?

 

The Cat itself can rattle if it falls a part inside. A bump on the cat(s) with a rubber mallet when its cold should tell you.

 

would it be noisy only on start-up or noisy the entire time the car is running?

 

Depends on how bad. Normally when they first fails its just cold. As the internals get hot and expand it goes away.

 

I would put it up on a lift and or jack stands to see what it might be.