Has your truck ever been wrecked? My 06 has the same issue. I took it to the Chevrolet dealership for the fan resistor recall and had them check it out while they were in there. A body seam within the outside airwell along the base of the windshield had separated and they recommended taking it to a body shop. Or, you can just turn on the A/C recirculate cabin air & it closes the door to the outside airwell, and any rain. I pushed 1 button and no more leak.