* Loose brake caliper: A loose brake caliper can cause the brake pads to move around and knock against the rotor.
* Damaged brake rotor: A damaged brake rotor can cause the brake pads to vibrate and make a knocking sound.
* Worn ball joints: Worn ball joints can cause the front end of the car to move around and knock against the brake rotor.
* Failed wheel hub bearing: A failed wheel hub bearing can cause the wheel to move around and knock against the brake rotor.