Noise when I step on the brakes making a humming sound.

[Street Fighter]

Each time I step on the brakes I am hearing a humming sound.I want to say it's my booster, but the brakes are still good and not hard.I have checked for a air leak but can't fine one.what could it be?

[Tomicich]

This is going to sound stupid, but I had this happen - rotate your tires.
I had the same symptom, sounded like it was for sure coming from the front of the car. I actually changed my front spindles thinking I maybe had bad bearings. A buddy told me to rotate the tires and the sound moved to the back until the wear evened out...

[pajeff02]

Depending upon the type of pads you can sometimes hear a humming or mild grinding noise when braking.

[SSnow]

Slotted rotors will hum while breaking. I tried them on a few vehicles a long time ago and abandoned the idea for a quiet ride.

[Street Fighter]

This has nothing to do with wheel movement, this is standing still. It is coming from the booster. If I remove the line plugged into the booster and place my finger over it there is no noise because their is no air going to the booster,I also noticed that with the line plugged into the booster when I step on the brakes the rpms increase.I believe that the booster is leaking once the brake pedal is stepped on,but I also noticed that it don't do it if I apply the brake hard sitting or moving.If I am right, how hard is it to replace the booster, just looking at it, it seems to be a job getting it out of the engine bay.

[pajeff02]

Sounds like a plausible explanation. I had ours unbolted (but not removed) for some of our installs, not too difficult to do. Fun part is always bleeding the lines afterwards.