I've heard with some brake pads, particularly performance pads, you will get some of the screeching sound. Forget the reason why, but that is what I have heard.
Sometimes it can also be the pads vibrating on the caliper.. to try to compensate for this, you can buy this red goo that goes on the back of the pads (the part that DOESNT touch the rotors) and it hardens up a little, sort of goes rubbery (i THINK), anyways, it is supposed to decrease the sound or get rid of it. I have bought some for my car, but can never be bothered ripping apart my brakes to do it. I've had it sitting there about 3-4 months now hahaha.