So, I randomly noticed a whistling noise in my LBZ on a trip. It seemed random, and I couldn't quite identify where it was coming from or why. Turned out we had set it to max A/C and the truck turned on the cab air re circulation feature to cool things down/keep them down. Anyways, on my truck when that occurs, it closes a "door" to intake fresh air and consequently air from the outside slams into that door and makes a slight whistling noise. Not saying its the same for you, but that is what I discovered on my whistling issue that "came and went" until I identified what it was.