Whistleing brakes on 2002 Volvo V70 XC

I have a hissing sound and whistle from (a braking) component. Reason, when I step (press) on the brake pedal the noises stop. Any clue as to what this is. It sounds like an air leak.

Asked by for the 2002 Volvo V70 XC
1 answer
brake booster could be leaking