Almost sounds like a fan blade hitting on something for about 1 to 2 seconds then it stops. Only happens after it sits up for a while.. Gonna check it after work, but anybody had this happen are any ideas?
I'd guess the obvious thing to check is the fan. With the truck OFF reach down where the fan is and see if there some play or wobble. If you are under warranty take to the dealership. A video on start up of what you are hearing would help tremendously.
The truck only has 45,000 miles on it. Vacuum pump shouldn't be going out and making a loud noise like that??? First thing, I'm going to check the radiator fan when I get home... But thanks for all the suggestions and keep them coming... Will post results later!!