My F-150 is doing the exact same thing. There is a whistle coming from the driver footwell that gets louder with the gear getting closer to changing. It does it on both acceleration and deceleration. It was not there for the first 800-900 miles and now seems to be getting louder and louder. It is not the electric motor or the backup alarm. This is clearly a whistle coming from the transmission. Dealer was no help at all. I think I’ll try to find a way to lift the rear end so I can get the truck to go through gears so I can try to pinpoint exactly where it is coming from. The whistle is driving me crazy and making me not want the truck anymore!