well, the only thing that i can think of is the blower motor as far as i can remember. The only other few things that i can suugest: tweeter (speaker)has fallen out of position.....a heat shield is loose, and that coudl be your rattling at certain revs....the more the truck shakes, the more it rattles...
i would more than likely say that it is the heat shields.... go to a dealer and ask them about it.. or do it yourself... part# is 15164621
and you will need 2 of them to go around the heat shield on the MUFFLER
best of luck