i had this issue, it was my cold air intake. they used a weird rubber bolt deal to brace it against the side of the truck and it melted in the heat and now rattles against the metal shield it passes through. just for giggles do you have a cold air intake? mine rattled just like your trucks symptoms.

 

Well, if a cold air intake is an add-on I don't have any. But I'll check that out. [I know, I know, my knowledge is zero]