i posted this in "general discussion", but no hits:

haven't really got underneath the truck yet, but on cold starts there is a rattling noise. sounds like it is some sort of heat shield rattling. it sounds like it is underneath the extended cab, but not sure. you can still here it as the truck slows down when approaching a stop sign/light. after about 1-2 min when the truck is warmed up, the noise seems to disappear.

what might it be?

 

Sounds like you have the famous Gen III piston slap. Nothing to worry about. My 2000 does it also and @ 93000 miles it is the same asit was @ 40000.