The sound starts as soon as I turn on my car and seems to vary if I accelerate or apply the brakes. Brake pads are good. It sounds like air rushing out of a tire.
It sounds like there's air coming into my car at the brake pedal on 2003 Audi TT Quattro
Not sure if your car is equipped with a vacuume assist power brake booster or a hydralic assist booster....but if the sound comes from the brake pedal area that typically means the vacuume booster is leaking air.
poss vac leak at booster if so equip