Muffler Rattle
 

When idling or coasting under about 5mph I get a high pitched rattle from underneath my truck. It goes away (or is muffled) when I give it gas. Using a make-shift stethoscope I've been able to pinpoint the rattle to the muffler. It looks like there is plenty of clearance for all parts of the exhaust, so I don't think anything is rubbing, and I tried banging on the cat to see if the internals had broken.

Could this just be the internals of the muffler shaking around? I'm worried that maybe this is a sound generated in the engine and it's just registering in the muffler, is that possible? I'm not sure if this is the original exhaust or not, but it's pretty rusty and the outer layer has been flaking for a while. I'm thinking now would be a good time to replace the muffler and tailpipe since I can get the dynomax cat-back for $65 after the mail in rebate if I order it before midnight. Any thoughts?

My cat is destroyed it rattles like crazy. Do the new exhaust thing like u said

Forgot to mention that this only happens once the truck is warmed up. Not sure what that might have to do with it

I had a 1-1/2 year old muffler let loose of a baffle and it would rattle around inside. Yours may be doing the same.

Probably a muffler bearing...

An off the wall thought here, but when's the last time you greased your u-joints?

My exact concern right now; well except generated within a different part of the exhaust. I am having the same problem as you and can only pinpoint it to the muffler. Rattles only at idle but goes away once I give it some gas regardless of in gear or not. I'm concerned that packing in the cat has broken free tearing apart the muffler.

How can I check the health of the catalytic convertor short of taking it off? I don't want to put on a new muffler just for it to be torn to shreds like the last one (if the cat is the case).

The same thing has started with mine. Only does it after it warms up. I only have 108,000mi on mine. Sounds like the muffler but I don't know for sure. I would like to know if anyone found out what it is. It happened shortly after I installed the APN manni.

About the only definitive way is to pull the cat and look. Or pull the muffler and look into the cat.
You could drill a small hole before and after the cat and perform a pressure differential test, but that will only uncover clogging, not necessarily mechanical failure. If you smack the cat with your fist and it doesn't rattle, chances are nothing is broken loose inside.

re: muffler noises:
If you hear a rattle or noise from the muffler...it's almost always the muffler. It's not likely it's an engine problem.
Mufflers are built cheap. Even the expensive ones.
Spot welds break, thin gauge baffles deform or detach, inner and outer shells vibrate. The heat/cool cycles, constant vibration and exposure to water vapor and other chemicals and gasses create a pretty hostile environment.