Would have to have someone get under it/take the wheel off to inspect. Could be many things:
Loose clips for brake hose etc, pads moving around, loose suspension components like sway bar link/bushings, suspension bushings, brake heat shield moving around or hitting something, exhaust rattling against something, exhaust heat shield rattle etc. etc.
Without an inspection, it would be hard to say. Also, ensure all your lug nuts are tight!