Full Article Here >> Well, I spent several hours on the internet via Google and discovered my problem was most likely coming from the right-rear cabin airvent which is located on the back of the cab metal wall. There is another vent on the left-side driver's side in the same area but it had no noise issue in the seven new crews I drove.
These vents allow airflow in and out of the cab for pressurization and operation of the climate control functions, defrost and etc... You will read about this as you read the articles via Google.
From the outside, if you look down between the bed and the cab you will see the black plastic frame of the air vent. Inside the plastic frame are semi-hard but flexible rubber louvre-type vents that move open and shut to allow airflow andair and road noise. Just think of a heat register in your home and then stand it on it's end--that's what we're talking about. The GMC vent may not look like these. But here is a picture so you will get the idea.