Rattling noise

I've got a '97 Blazer LS, 4.3, 2wd

There is a rattling noise-sounds like metal rattling against metal-coming from the underside of the truck.
Only seems to do it when the truck is sitting still & in gear. Seems to be a little louder in reverse than drive though. Goes away when I release the brakes & start rolling.

I'll mention this in case it's related...lately I also have an occasional "clunk" when first taking off. I think that may be a u-joint but I haven't had time to check those yet.
Also had the rear main oil seal & oil pan gasket replaced not too long ago, so I'm wondering if the rattle is related to a motor or transmission mount (?) that was loosened to get to those jobs.

Any idears?

-Mike

Rattle: Cat Convertor

clunk: u-joint- probably the rear one.

Ok, that makes sense...so another question...if I understand correctly, the cat generally shouldn't fail unless something else is wrong (?)

Other than the aforementiond "clunk" which shouldn't affect the converter, the Blazer has been running great....would it make sense to go looking for the cause of the cat rattling, or just wait & see what happens?

Thanks!
-Mike

I had a rattle like the one you described, it was the the skid plate that covers the tranny and transfer case. One bolt was missing and the others were loose.

I think the cat rattle he was refering to is a heat shield on the cat that has broke loos since it is only spot welded together or of course it could be some broken catalyst inside the cat.