Whistling noise from beneath hood when accelerating/ stops when brake is applied. on 2001 Mazda Tribute

I have a high pitched whistling (think Tea kettle) that occurs, randomly when the vehicle has been running for a while, or may start right off the bat. The whistling will stop when the brakes are applied and will not start again until you accelerate. no issues with braking or acceleration performance. Mechanic said - no safety issues but doesn't know what is causing the noise. Anyone have any ideas?

Asked by for the 2001 Mazda Tribute
I had this exact same problem. It turned out to be the intake manifold gasket that needed replacement.
yep... took it to another shop and that's what it was. thanks for the reply
how much did that cost?
Wondering the same thing, how much?
1 more answer
This is probably related to a vacuum leak. It may be the brake booster, so have your mechanic disconnect and plug the booster vacuum hose and see if the noise goes away (do not drive with this hose disconnected). Check all the vacuum hoses on the engine as well. Let me know what you end up finding.