A CRJ200 ON CLB AT 15000 FT; HEARD A SMALL THUD. ALL ENG; HYDRAULICS; AND FLT CTL SYSTEMS INDICATIONS WERE NORMAL. ON GND; INSPECTION FOUND L ENG EXHAUST NOZZLE AND FAIRING MISSING.

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UPON PASSING APPROX 15000 FT; HEARD A SMALL THUD. IMMEDIATELY CHKED BOTH ENGS. ALL NORMAL IN GREEN BAND/ARCS. VIBRATION GAUGES L=0.2; R=0.4. ALL SYSTEMS (ELECTRIC; HYDRAULIC; ECS; ANTI-ICE; FLT CTLS; ETC) WERE NORMAL. FLT ATTENDANT CALLED AND ALSO HEARD SMALL THUD. TOLD HER WE WERE BUSY CHKING SYSTEMS; BUT SO FAR EVERYTHING WAS NORMAL. WE WOULD CALL HER BACK LATER; WHICH WE DID. WE HAD NO WARNING OR CAUTION MESSAGES. I WAS VERY SERIOUSLY CONSIDERING RETURNING TO PHL; HOWEVER; DUE TO THE FACT THAT ALL ENG INDICATIONS WERE IN THE GREEN RANGE; WE ELECTED TO CONTINUE. ALSO CONSIDERED THAT WE HAD MANY OPTIONS: LOTS OF FUEL (FUEL AT LNDG ESTIMATED 3900 LBS); MANY DIVERSION ARPTS ENRTE; AND WX WAS VMC. WE ALSO THOUGHT IT COULD HAVE BEEN WAKE TURB FROM A PRECEDING ACFT; OR OUR PLANE'S CREAKING NOISE AS CABIN PRESSURE NORMALLY INCREASES AT CLBOUT; ETC. L ENG: ITT 770 DEGS; FF 1450; VIBRATION 0.2. R ENG: ITT 802 DEGS; FF 1550; VIBRATION 0.4. WE CONTINUOUSLY MONITORED THE ENG INSTRUMENTS FOR REMAINDER OF THE FLT. AFTER LNDG IN ZZZ; I DID THE POST FLT AND NOTICED L SILVER TAILPIPE MISSING. IF WE HAD ANY INDICATIONS IN THE COCKPIT OF THIS; WE WOULD HAVE RETURNED TO PHL IMMEDIATELY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED AFTER THE THUD WAS HEARD; THE INSTRUMENTS WERE SCANNED AND EVERYTHING WAS NORMAL AND NO VIBRATION INDICATIONS FROM ENG VIBRATION MONITORS WERE NOTED BEFORE AND AFTER THE LNDG. THE RPT FROM MAINT ON THE MISSING EXHAUST NOZZLE AND FAIRING INDICATED THE NOZZLE IS FASTENED TO THE AFT TURBINE MODULE WITH 65 RIVETS. THE RPTR INDICATED ONE OR TWO ADJACENT RIVETS FAILED AND ALL OF THE RIVETS FAILED PROGRESSIVELY AROUND THE TURBINE MODULE AFT MOUNTING RING. RPTEDLY THE NTSB HAS RECOVERED THE NOZZLE AND FAIRING.