The most common reason for brake grinding noises is thin brake pads that press the pedal causing contact between the rotor and the metal backing plate of the pad. If the vehicle has brake shoes on the rear, a similar condition causes this metal-to-metal grating noise. CarsDirect notes that brake grinding is a serious problem that requires immediate attention.
The grinding noise that you hear is the wearing away of the brake rotors or drums. Each time you apply the brakes, more metal wears away. With a normal brake job, the technician turns the rotors on a lathe to create a smooth surface for the pads to stop the vehicle smoothly. Brake manufacturers include a wear indicator in brake pads. This indicator is the reason for the high-pitch squeal you often hear when a vehicle's brakes are applied. However, if you ignore the warning indicator, the pads continue to wear away until they damage the rotors. Replacing pads when the indicator squeals protects rotors, prevents expensive damage and ensures the vehicle stops correctly.
In rare cases, the grating noise is due to a rock or other particle caught between the brake pad and rotor. This noise usually continues even when you release the brake pedal. It also requires immediate attention.