What was found
TRIP NR 1152283/ CREW REPORTED AT FLIGHT LEVEL 380 IN CLEAR SKYS AND SMOOTH AIR A LOUD NOISE SOUNDED AND RANG OUT THROUGHOUT THE AIRCRAFT. AIRCRAFT PITCHED DOWN MOMENTARILY AND CREW DISENGAGED THE AP AFTER WHICH THE AIRCRAFT RETURNED TO NORMAL LEVEL FLIGHT. MAINTENANCE IS INVESTIGATING THE DISCREPANCY. MAINTENANCE PERFORMED AN EXTENSIVE INSPECTION OF THE FLIGHT CONTROL SYSTEM, TRIM SYSTEM, AND THE AUTOPILOT SYSTEM. NO DISCREPANCIES WERE NOTED. THE DISCREPANCY HAS NOT RETURNED.