Noise from the Brakes

I recently bought a 2000 Mazda 626 LX 2.0 Liter engine, with 112,700 miles on it. The rear brakes were replaced as well as the calipers. However, when I barely press on the brake pedal a noise seems to be coming from the brakes. Did the mechanics who worked on the brakes put the calipers or brakes on wrong? Or could it be the rotors? Thanks