there are 2 things that could cause the sound. If the car has power brakes the sound could be l leak in the vacuum booster or just the sound of the diaphram in the booster as it applies the vacuum to the system. the other cause could be worn brake pads and the sound is actually from the rotors against the pads. However, the brakes are the most important part of your car when you are moving. I would have a local brake shop investigate for your safety and the rest of us who are driving near you.
