On the road all day today I started hearing a hum in the transmission or transfer case like the truck was locked into 4x4 and running at freeway speed. I stopped and checked the fluid level in the tranny and it was fine and didnt smell burnt. On down the road a metal on metal noise started developing, sounds sort of like light metal rubbing on the driveshaft. I stop again and crawl around all over under the truck and find nothing. Back on the road about 50mi. down the road there was a huge poping/grinding for a few secconds similar to the sound of a drywall screwgun clutch but WAY louder. Pulled over looked all around and found nothing. Started back down the road with the metal on metal noise, 50 or so miles down the road POP CRUNCH GRIND. About every 50 miles or so Im getting the same poping grinding and it goes away after a minute. I tried 1st, 2nd,3rd,Drive, and neutral. 2 and 4x4 and the same metal on metal sound is still there.

Seems like a trans, or transfer case problem. Aside from the god awefull sounds its making the truck is driving just fine. Truck is an 03HD

Ideas?