You have a vaccum leak. When you step on the brake, vaccum is being used to help the power assist, thus lessening the amount in the engine available to leak. The reason it goes away when you accelerate is that vaccum drops low on acceleration too.
he he he
Brakes
You might have a belt loose, which is more of a squeaking noise than a whistle. If it's a very clear whistling noise, there is something wrong with your brakes. The shoes may be rubbing, the pads may be worn down. I'd get it checked out.
it means your brake pads are worn out and need to be replaced
the whistle
When you are driving slowly with your foot off the brakes you will hear a scraping noise. The noise will go away when you apply the brakes. What you are hearing is the wear indicators making contact with the rotor surface after the pad lining have worn down to a certain level. Not all brakes uses wear indicators. VBD
it could be a bad wheel bearing.
Numerous possibilities, wheel bearing, brakes, belts, etc.
Check front pads and brake rotors. You might have to change the pads.
did you check your brake pads, to me it sounds like you need new brakes, i imagine that if you have been driving with that noise happening and you didn't replace your brakes your rotors are most likely scored, possibly warped, full of hot spots, and heat cracks
toot
this is usually the indicator that you need new brakes so dont worry just get them replaced problem solved