Different things can cause brakes to make a grinding sound, including severe wear to the brake pads or something coming in contact with the brake caliper. As the pads become wore down, the caliper comes into direct contact with the rotor.
If the brake grinding is caused by worn down brake pads, immediate repair is vital. Once the brake pads become completely worn down, it can cause brake failure. Driving with worn down brake pads can also cause the caliper, drum or rotor damage which will end in costly repairs for the car owner.
Another less common reason for brakes making a grinding noise is something becoming lodged in the caliper. When this occurs, the brakes may continue to grind when the vehicle is in motion and the brakes are not being used. If the object is large enough, or stays in the caliper for an extended period of time, damage to the pads and other components is possible.
Other types of unusual brake noise includes rattling, squeaking or vibration. The most likely cause for all of these noises is worn down pads, but warped rotors, a bad caliper and other things can also cause this noise. All brake noises should be checked out by a certified mechanic for safety reasons.