Rigged flaps about 15 years ago. This past year when extending the flaps from full up I heard a small CLUNK. With time it became more noticeable and suspected the limit switch was out of adjustment. It was. Spent the whole day re-rigging the flap system. The clunk is from the windup torque in the flex shafts and also suspect this may be the cause of extra load on the flap rib cracking problem. So listen to your airplane, it talks back.