Brake quacking - something stuck?

Recently whenever I stop the car with a foot on the brake, I hear this intermittent quacking sound. Initially I thought someone in another car was lightly tapping the horn. Then I realized it was my own car. It sounds like something got stuck and then briefly got unstuck and made a sound. It doesn't happen every time. It could happen when just releasing the brake also. What could it be? The car drives and brakes normally as far as I can tell. The accumulator sounds the same as before (definitely different from this new sound).