i have this same issue (rattles in D, and sitting still) but i found the cause to be something else...

the driver side exhaust pipe is VERY close to an aluminum cross member or something and its rattling on that, so by moving it out of the way slightly it stops.

 

Thanks for the help ill check that out tommorow

 

ill take a pic of what i mean after work and post it

 

My truck DID that, there was an identification plate in between the cat and the heat shield and would rattle like a MOTHER ****ER! I got under the truck with a mallet and beat the cats cause i thought it was the shield as well. Didn't see it untill I was crawling out from underneath and it caught my eye, it was about 2inches by 3 inches little identification tag for the cat. I pulled it out with a pair of needle nose pliers and VIOLA! the noise went away.

 

I had 1 with the tag/id plate rattle as well. Also had a spot weld on the heat shield pop once and cause a hell of a rattle when stopped in gear.

 

Thanks ill check tommorow when it isnt raining. Ill let u know wat i find

 

Okay so i just checked it out theres no tags no cross members no nothing touching it. Im stumped. Guess the only thing to do is replace the cats uhg. Btw i have a 4.6 in my truck

 

Geeze, I sure had hoped it was just a loose shield or something easy.
Cats aren't cheap.
When in doubt, hit it with a hammer.
Try rapping on the cat and exhaust parts with a rubber mallet.
Perhaps you could isolate the rattle more closely.
Maybe just a piece of junk in a pipe.