OMG, I had this *exact* problem on my 1500, especially when it was cold.
Turns out it was the hood bushing - little pieces of black rubber that keep the hood from "falling" too far into the engine bay.
Greased up both bushings with a liberal dab of silicone grease and it's been silent for over a year since.
The bushings are on either side of the hood, up from the headlights on the "body rail" area. They keep the hood from contacting the body metal to metal. They're really quite hard, especially when it gets colder.