Does he have a bug guard on the front of the truck?
This may sound stupid, but it's true. With no bug guard on the hood, the air flows over the hood and around the windshield properly and no noise. A bug guard disrupts the air flow over the hood and causes the side window trim rubber to "flap" or vibrate a little in the wind causing the rain or flapping sound you're probably hearing. I tried this on my truck (bug guard vs no bug guard) and with the bug guard on there's noise, off, no noise.
Here's a fix for it:
http://www.gm-trucks.com/home/Techni...ug-shield.html
Post Number 6 in this thread is interesting too:
http://www.gm-trucks.com/forums/inde...ield+noise+fix
I've actually never done anything to fix mine... I kinda like loud music...
Edit: What year is the truck, I think this only applies to the 99-02 silverados and 99-07 classic sierras, I don't know if the 03+ silverado's new square nose caused this sound or not?