Is it making the SAME screeching noise as it was before you changed the pads? Is it possible that the rotors have a sharp lip along its edge? or....maybe this high pitch noise is coming from the engine bay? if so, check your belt and belt tensioner.
Same problem on our 2009 Rogue. I bled the brakes and everything has been great since then. A tiny bit of air in the caliper can cause it. As the wheels get hot from braking, the air expands and slightly activates the brakes, making the problems worse.