Aloha from Hawaii! I have a 2000 4WD, 4-door Blazer. Love her to death. She has about 126,000 miles on her. I have changed out the fuel pump/relay/filter already, so I know it's not that.

Here's the issue: about a week ago I noticed a rattling coming from below, more on the passenger side. It's a little muffled, as if it is coming from inside a part (versus a loose heatshield), and sounds a bit like marbles in there. It has gotten louder over the past week, but I only hear it when the vehicle is in gear and not in park or neutral.

I thought it could be the cat converter, but I think any perceived loss of power has more to do with my imagination after reading about that online. CEL recently came on, but didn't get to take her in for a reading secondary to the three-day weekend. All other fluids seem fine, only other replaced parts were the A/C relay and master brake cylinder. Banged around everything below with an old flashlight, but couldn't reproduce the rattle.

Any ideas, anyone? Much mahalo (thanks) in advance!

 

Possible low octane fuel causing pre-detination when engine is under load.

 

My catalytic converter rattled really loud under load. Didn't rattle while idling. I never had any loss of power or CEL. I finally figured out the rattling by gently pressing against the exhaust outlet with my foot with the Blazer running, it would rattle depending when I did that. Let us know what you find out please!

 

Thanks all! The shop said it was the cat, and based that on the sound of the idle and the rattle produced with a good mallet whack. I had a hard time getting her to start tonight after work, almost sounded like the time the fuel pump died. I'm thinking that the broken pieces are starting to clog up the exhaust, but she never died on the road and I didn't notice a loss in power. Is it a bolt-on piece? I've seen the cat alone online for ~$50, but it looks like I gotten cut & weld it into place. There was a thread here that discussed removing it, but it was a violation of the forum rules...

 

Truck had difficulty starting up, turns over a lot before she gets going; took off the heatshield and holesaw'd the cat, chunks came out. Must have been clogging up the muffler. I broke out the pieces and shop-vac'd what was left. Couldn't patch up the hole beyond sheet metal and pop rivets, my baby now sounds like a half-assed Bubba'd Flowmaster job and smells like my old man's lawn mower. Hell, maybe ten years ago I would have thought it sounded cool...