I noticed this when closing my door. I'd hear something rattle under the truck.
Then it rattles on bumpy roads like when we adventure out on gravel.
Last weekend it was really bad so I finally got under the truck and its the exhaust. At least I think it is.

I started banging with my palm from the cat to the y pipe split and I get that rattle no matter where I hit it.
I couldn't say definitively it was near the cat.
Nothing seems actually loose.
Doesn't rattle under idle or normal driving.
No codes, seems to run just fine.

Any ideas?

 

loose cat element.....

 

Loose heat shield?

 

Check the heat shield on the cats. I've seen those welds break before and cause a rattle. Lay hands on every bolt I'm the exhaust system, make sure they're not loose. Double check clearance between the exhaust and everything surrounding it, I've had rattles caused by things (such as the frame) coming in contact with the exhaust.

If all that checks out, either the inside of your muffler is rusted out and rattling around or the converter element is loose/melted and the cats will need replaced. If it's the muffler you should see other signs of significant corrosion on the exhaust system. I doubt it is the muffler.

 

Yeah, had to weld the small heat shield behind cat on my 01...drivers side.

 

I had 2 magnaflow cats that both started to rattle from bad elements

 

Turns out its the weld on the thin heat shield on the drivers side cat.