Brakes squeak 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. Braking noise can occur for other reasons than worn out pads, though. Moisture can cause brake rotors, which are made of unfinished metal, to rust as quickly as overnight. This is normal, which can lead to a grinding noise, and will last until the surface rust is removed. Hard braking creates significant heat, which can glaze brake pads and cause a squeaky brake noise. Also, brake pads designed to for endurance are made of a harder material and can emit noise as part of their normal operation.