Squealing brakes can be caused by a variety of factors which includes The presence of grease or oil on the brake pad, rotor, or wheel rim, or a misaligned contact between the braking surfaces. Manufacturers of brake pads typically specify a minimum thickness for the pad material. The brakes may squeak if the thickness of your brake pad is less than 1 mm. Newly installed brake pads also need time to bed-in for better performance.