What was found
PILOT REPORTED A 'POP' WAS HEARD WHEN SELECTING 4TH NOTCH OF FLAPS ON LANDING. POST-FLIGHT INSPECTION WAS UNABLE TO LOCATE SOURCE. AT NEXT TAKEOFF WITH FIRST NOTCH OF FLAPS AIRCRAFT ROLLED TO THE RIGHT. FLAPS WERE RETRACTED AND SAFE LANDING ACCOMPLISHED. IN-DEPTH INSPECTION FOUND BRACKET UNDER CABIN FLOOR BROKEN. FLAPS WORKED NORMALLY, HOWEVER, WHEN FORCE WAS APPLIED TO RT FLAP, IT MOVED UPWARD APPROXIMATELY 15 DEGREES.