Braking System

That is usually the sensor starting to tough the rotor.
There is a little steel arm that is on at least one if not both brake pads. When the pads need replacement, they start to touch the rotor and make a, "squealing" noise.
If you replace the pads soon, the rotors still might be within service limits. However, if you wait the squealing will turn into grinding and the rotors should be replaced.
I hope this helps.