I've had this problem almost since the truck was new and heck I thought I would throw it up here. It hasn't posed any issue that I know of other than being annoying.
Basically when the truck is cold it will whistle from the engine. The pitch goes higher and lower with RPM. I thought maybe it was something in the timing belt but even after I changed the timing belt and waterpump it still did it. I'm pretty sure this is an air noise and not a belt noise as changing the belts made absolutely no difference. Almost like the intake manifold is sucking air when it's cold or something.