If I am at a stop sign and put the truck in D I get a rattle from the back - I'm pretty sure - for about 10 seconds and it stops. Won't do it in N but I can repeat it over and over if I want. What is it? Why only 10 seconds? Just put a new muffler on so that wasn't it.

 

Had a similar problem, it was indeed the tail pipe. Especially since you got a new muffler on, they could have skewed the angle just enough for the tail pipe to be making contact with the frame somewhere. Check clearances towards the output of the tailpipe.

 

I'll give it a shot