A CRJ-700 FLT CREW NOTED SUBTLE YAWING ON CLB AND; LATER DURING DSCNT A LOUD 'THUD' WAS HEARD. ON GND INSPECTION FOUND THE L INBOARD MAIN LNDG GEAR DOOR MISSING.

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DURING CLB OUT OF ZZZ; I NOTICED A SUBTLE ACFT YAWING FROM SIDE TO SIDE; ABOUT WHAT YOU MIGHT FEEL IN LIGHT CHOP. WE CHKED ALL SYSTEMS; AND AT THE TIME; I THOUGHT MAYBE THE YAW DAMPENER WAS CAUSING THIS. BEFORE WE DID MUCH ELSE; THE SLIGHT MOVEMENT WENT AWAY. WE HAD JUST MADE A TURN FROM APPROX 210 DEGS TO DIRECT ZZZ1 ON COURSE. NO OTHER ANOMALIES WERE NOTED. DURING OUR DSCNT OVER ZZZ2; WE FLEW INTO THE CLOUD TOPS AND SIMULTANEOUSLY FELT A THUD UNDER THE ACFT. I CALLED THE FLT ATTENDANTS AND THEY ALSO HAD HEARD THE NOISE. IT SEEMED TO HAVE COME FROM UNDER THE GALLEY; ACCORDING TO THEM. I CALLED MAINT AND THEY MET THE ACFT. THE INFLT NOISE AT THE TIME WAS THOUGHT TO HAVE BEEN MAYBE COME FROM THE FWD CARGO; FORWARD SLIDING TRAY NOT BEING LOCKED IN PLACE. WHILE THE MECHANICS WERE INVESTIGATING THAT POSSIBILITY; THE FO ON HIS WALK AROUND DISCOVERED THE MISSING L INBOARD GEAR DOOR. I'M NOT EXACTLY SURE NOW WHAT CAUSED THE NOISE DURING THE DSCNT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE FWD L INBOARD MAIN LNDG GEAR DOOR HINGE HAD FAILED. WHEN INFORMED BY THE ANALYST OF THE AIRWORTHINESS DIRECTIVE ON THE INBOARD LNDG GEAR DOOR INSPECTION REQUIREMENTS; THE RPTR STATED; 'YES; BUT THE NOTE ONLY COVERS DOOR STRUCTURE AND NOT HINGE CRACKS OR HINGE FAILURES.' THE PROB IS THAT DURING A WALK AROUND INSPECTION; THE FWD DOOR HINGE CANNOT REALLY BE SEEN.