These are the things that make you hate mechanics
![Hammer :hammer:](https://board.marlincrawler.com/Smileys/marlin/hammer.gif)
Try checking the brake booster. Mine once had a similar problem that was caused by a pinhole leak in the brake booster, so under a vacuum the air would whistle through the hole making a high pitched noise. but unlike yours, you could hear it whenever the truck was running. Thats my
it might be worth checking out on yours.