More guesses: I have had a knocking sound because of a loose mounting bolt on the brake caliper. The two parts of the caliper have to slide together as the brake pads wear, and they move back and forth on the mounting bolts. Either bolt being loose (one of mine was stripped) can let the caliper contact the inner part of the wheel.
Could the noise be loose lug nuts? ..or a loose hub nut? ...or a bad wheel bearing?