Sometimes My car has got.

You'll have to hold your computer closer to the car so I can hear the noise.

You could be hearing the hiss of air entering the power brake booster when you push the brake pedal. You could be hearing the front pads rubbing on a light film of surface rust that normally builds up on the rotors overnight. You could be hearing the rear shoes sliding on the backing plates as they release. Higher-quality brake linings are real hard and make a grinding sound until they warm up. You need to visit the dealer and have a mechanic listen to the sound. The first thing he is going to ask you is where you're hearing the sound from.