Wind is definitely a factor. I-5 to and from LA was extremely windy. Every time the wind blew harder the pitch would go up. Even with windows open it makes the sound. I used the wipers to see if the rubber blades where causing it but nope, not the wipers. So it would have to be the mirrors, or some body component or the windshield. Another thought is the spacing between the windshield seal and the metal frame. There's a trough to drain rainwater. I can see that being whistler.