Keeping the wheels straight, move forward, apply brakes and come to a full stop. Keeping the wheels straight, put the car into reverse and then hit the brakes.
Do you hear the clunk then?
Put the car in gear and move forward and apply the brakes again. Does it clunk?
I had an issue on my B8 S4 with the stock floating caliper in which the carrier bushing had becomes soft enough that it allowed the caliper to move if that makes sense.
Not sure if that’s the culprit here (or if you even have a floating caliper). Just trying to contribute.