A rattle, clunk, or buzz from the brakes when you go over bumps - but not necessarily when braking - is a distinct and common noise. It usually means a brake pad has room to move around in its bracket. Here is what causes the gap and how to quiet it.
Loose pads from missing hardware
Brake pads are held snug in the caliper bracket by anti-rattle clips and springs - part of the brake hardware kit. When those clips are worn, corroded, or were not replaced at the last brake job, the pad can shift slightly and rattle against the bracket over bumps. This is the number-one cause of brake rattle - and exactly why hardware should be renewed with every pad change; see when to replace brake hardware.
Missing shims and worn abutment clips
Missing or flattened shims and worn abutment clips let the pad sit loose and vibrate. Rust building up behind an old clip also changes the fit so the pad no longer seats tightly. The result is a metallic rattle that comes and goes with road bumps.
Other rattle sources
Not every brake-area rattle is the pad - a bent dust shield touching the rotor or loose caliper bolts can rattle too. But if the noise is a light clatter over bumps that quiets when you press the brake (clamping the pad tight), the pad/hardware gap is the likely culprit.
The fix
The cure is almost always fresh hardware: new anti-rattle clips, shims, and springs seat the pad snugly and kill the noise. Clean the bracket of rust so the new clips fit properly. Install quality pads with a complete new hardware kit and the rattle goes away.