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A grinding and growling sound of metal on metal means that the brake pads are completely gone. It is advisable to have a certified brake repair technician inspect your brakes on an annual basis, twice a year if you regularly do heavy hauling and towing work. Your brake repair technician should also inspect all metal to metal contact points to be sure that they are properly lubricated (and maintaining an effective barrier between the contact points) and inspect your brake fluid to be sure that there is no contamination from metal flakes or moisture. Fluid contamination can cause damage to essential braking components. And speaking of fluids, it is recommended that you not top your brake fluid off whenever the level goes down.