Take a look at the pic, you can see where the dash meets the windshield glass in the right corner, the dash is flared rising about maybe 3/4". If you press on the corner of the dash here it can be pushed down, but it will not stay down. It's like looking at a picture on the wall that is...
Agree with others potentially heat shield, does the volume of the rattle change
at all while in/out of gear? Just get under the truck and poke your heat shields, you'll know in a second if these are causing the problem.
Suspecting a wheel bearing here too.