Squeaky brakes can occur for many reasons. Most brake pads are designed to start making noise when they are worn down to the last 25%. This squeaky or squealing noise can be an indicator that it is time to have your brakes inspected and possibly replaced. Brake noise can occur for other reasons than worn out pads, however. Moisture can cause brake rotors, which are unfinished metal, to rust as quickly as overnight. This is normal, and can lead to a grinding noise, which will last until the surface rust is removed. Hard braking creates heat, which can glaze brake pads and cause a squeaky brake noise. Also, long-life brake pads are harder and can emit noise as part of their normal operation.