Well, I finally found the source of the noise.
73fxe was right. Turns out, the windshield was replaced at some point before I bought the truck.
Found the after-market sticker.
Eventually I went to my mechanic and took him for a test drive. Of course, for the first 10 minutes, NO NOISE.
, then it started very quietly but within a few minutes it was going full blast.
The source of the noise is either the trim or the sealant around the top of the windshield. I still need to take it to a glass shop and have them removed, clean and reinstall the windshield or replace it if it breaks while the attempt to remove it.
I can't believe I've put up with this for two years. Arghhh. Well, anyway, thanks for all your ideas.