(2000 Blazer, 4D, 2WD, 6cyl, 150k miles) Rattles when started. Turning off A/C during warm-up makes the sound go away or at least reduces it to a muted rattle. During idle, goes away quicker if driving. Seems to be coming from the front of the engine (I initially thought the hood brace was loose). No engine hesitation or otherwise. Do you think it's the tensioner assembly? It's been going on for a long time, and still passing emissions test, so I assume catalytic is still kicking.

Check the belt tensioner and idler for loose bearings, and check the exhaust for heat shields that are corroded. Also might be a loose engine mount bolts.

Check the alternator bracket mount for loose bolts. Had that rattle on my 2000 also.

Simple test for a bad Cat, Bang on it with a rubber mallet and see if it rattles.

Simple test for a bad Cat, Bang on it with a rubber mallet and see if it rattles.

Good idea! Lemme give it a go

Ok, question: The bolts look good, but the tensioner pulley fluctuates when engine is running. Should it hold strong and not move during operation, or should it bounce slightly like it is?

Banged on catalytic, no noise from it, though the whole exhaust pipe vibrated and a slight rattle against the frame.

It will move around a bit but not too much.

There should be at least an inch of clearance between the exhaust and any part of the frame.

Ok, got the new tensioner in the mail. Installed, rattle SEEMS to be gone after a few days. I did here it once, briefly, but very muted compared to before. Otherwise, it's silent and appears fixed. Thanks for the help guys.