I recently bought a used 2000 Chevy Express 1500 with Vortec 4300, 4 spd transmission and 63K, having been a former Dodge Ram Van fan ( not an option any more ). I like the Express, however, I seem to have a rattle or rumble on the passenger side, only when then vehicle is idling with a load on the engine such as in drive with the AC on and headlights. It sounds like a bunch of rattling tin cans. I checked the exhaust system and it seems to be tight, but does seem to shake with engine firing at low engine idle speed. Any one have any ideas what is causing this and how I can fix it?

check all the heat shields...

your problem is your catalytic converter.

It's probably the exhaust donuts worn thin. These seal the exhaust manifolds to the exhaust pipe. underneath the engine. They're not too difficult to replace.

id deffiently have to say heat shields on that one but what do i know just been a mechanic for 5 years