Strange Rattle

My sons 1999 blazer has recently produced a strange rattle when idling. It seems more prominent under the vehicle on the passenger side. It is very inconsistent in rhythm and can't be heard when revving the engine. It will rattle when put in drive but goes away when accelerated. I've crawled under and did the touch and feel and look and listen without any luck of finding this noise. has anyone else had the same mystery?? Any suggestions would be great. Thanks

Check your catalytic converter. On my truck, the catalyst "chunk" inside the converter body has broken loose and rattles when the truck is idling in park/neutral. When I put it in drive, it sometimes gets worse or stops rattling. When I am driving I almost never hear the rattle, but my exhaust tone sometimes changes which I assume is the "block" has changed position. I am waiting until I need to pas the E-Test next year before I change mine, unless I come across the $300 required to buy a good name brand replacement converter.

When your truck is cold and shut off, bang on the converter body. If you hear a rattle inside, you found your problem.

Thanks, I'll check it out.

I have almost an identical problem with my 97 S-10. Unfortunatly I have yet to identify the cause but I am currently troubleshooting several things . I will be sure to let you know if I make any progress. First the fuel pump , steering gear mounting bolts, oil pump,oil pump drive gear .. eventually to things like strut mounts/ball joints/rocker arms. Good luck!