What is making the grinding sound in my Lexus brakes?

The loud metallic noise when braking that you will hear means that the brake pads have been completely worn down. The growling or grinding noise is being caused by the friction of the two pieces of metal (the caliper and the disc) rubbing together. This can scratch or "score" the rotors, making the surface uneven. When this happens, it is likely that your Lexus mechanic will tell you that both the rotors and the brakes need to be "turned" (evening out the surface of the rotor), or possibly even replaced.