I also think it's most likely the electric ABS/booster pump motor making the noise, if it is the motor it should also make the noise when you use the brakes several times, normal operation is the motor comes on when you turn on the ignition and also comes on after you depress the brake pedal approximately 3 to 4 times as the pressure drops, the motor normally runs about 10 to 15 seconds or so the motor should only make a slight buzzing noise but if the brushes are worn down it will be much more noisier like the noise you're is making.

You need to pinpoint where the noise is coming from, if in the area of the master cylinder, then I would check the motor.