Some rear disc brake pads are equipped with a metal tab which will come into contact with the disc after the friction surface material has worn near its usable minimum. The wear indicators make a constant, distinct metallic sound that should be easily heard. (The sound has been described as similar to either fingernails on a blackboard or a field full of crickets.) The key to recognizing that it is the wear indicators and not some other brake noise is that the sound is heard when the truck is being driven WITHOUT the brakes applied. It may or may not be present under braking during normal driving.