What year?
I've had a wind draft on my 99 Silverado for years I haven't really figured out. Comes from the passenger side, near the windshield, door, dash area, around 70-75 MPH+ depending on how windy it is outside. My suspicion is wind going through the fender and into the door somehow making it into the cabin. Not sure though. I removed the cowl cover a few months and didn't see anything. Windshield was replaced because of unrelated crack, and no difference.
My plan is to at some point add the little baffles between the fender and doors GM apparently added at some point...Fenders have to be removed for this.
My 06 Suburban doesn't suffer from this so I'm using that one as a control, if you will.