PontiacBonnevilleClub.com

I heard a rattling noise today and took the car to my mechanic. He thinks it the catalytic converter, but I only hear the noise when the car is in drive. When I rev it in neutral everything sounds normal which leads me to think it has something to do with the transmission. Any ideas?

More than likely it is the Cat starting to break up. I experienced the same thing when mine started to go. The material inside the converter deteriorates and eventually will get small enough to rattle around inside the converter body. Mine twisted around until it got wedged sideways and blocked the exhaust from leaving the converter.

For me it happened around 160K.
You only hear it at idle because there's less exhaust pressure flowing through the cat at idle...

I only hear the rattling when i first take off, still the cat?

I would say yes...