So ive had this sound since i got the car and it never went away (after full pad and rotor swap).. It happens when i press the brake pedal down a little bit and the car rolls slightly forward, the brakes grab but make a weird whale sound... like a low pitch moan.. idk thats the best way to describe it, it sounds like the pads are grabbing, but the rotor is still moving so the pressure on the pad and the forward moving rotor make this ... moan like u hear on the discovery channel from a whale ( ) the brakes work fine but i just want to hear that some one has the same problem and im not crazy... its loud too, louder than any other car ive driven.

 

check all of your caliper hardware. make sure everything is lubricated and not sticking or worn.