A clunk or knock that happens the moment you press or release the brake is different from a squeal or grind. It usually points to something loose or worn moving under load. Here is what causes a clunk when braking.
Loose or worn brake hardware
Missing or worn anti-rattle clips, or a pad that can shift in the bracket, can clunk as the pad seats against the rotor; see caliper bracket. Loose caliper bracket bolts or a worn slide pin let parts move and knock under braking load.
Suspension and steering parts
Braking loads the suspension, so a clunk when braking can actually come from a worn ball joint, control-arm bushing, sway-bar link, or tie rod - not the brakes themselves. These often also clunk over bumps; a clunk both braking and over bumps points here.
Clunk when turning too
If the clunk happens braking AND turning, it overlaps with steering and CV-joint causes; see clunk when turning. Isolating exactly when it happens helps pin down the source.
What to do
Have the brake hardware, caliper bolts, and slides checked first, then the suspension joints. Tighten to spec and replace worn clips; see pedal vibration. Fit quality pads and fresh hardware.