FLC OF LTT HEARS LOUD BANGS ON CLB. FLT ATTENDANT RPTS SMOKE IN CABIN THEN FLC NOTICES SAME. RETURN LAND.

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DEPARTED MOB FOR FLT TO HOU. CLBING THROUGH 8000 FT WE HEARD A LOUD BANG FOLLOWED BY 2 MORE IN RAPID SUCCESSION. THE FLT ATTENDANT CALLED AND RPTED SOME SMOKE IN THE CABIN AND WE WERE STARTING TO GET SOME IN THE COCKPIT. NOTIFIED MOB APCH OF EMER SIT AND REQUESTED VECTORS FOR LNDG AT MOB. THE FO (PF) HAD REDUCED PWR TO AROUND 50 PERCENT TORQUE AND WAS MAINLY INTERESTED IN FLYING THE ACFT. I WAS LOOKING AT THE ENG INSTRUMENTS AND NOTED LARGE FLUCTUATIONS IN THE L ENG; AND ALTHOUGH NO FIRE WARNING WAS ON; THE PRESENCE OF SMOKE HAD US THINKING ABOUT AN ENG FIRE. THE MEMORY ITEMS FOR BOTH ENG SHUT DOWN AND ENG FIRE HAVE THE SAME BEGINNING FEW ITEMS. THEY FEATHER AND SECURE THE AFFECTED ENG. WHILE DOING THESE ITEMS; THE SMOKE WENT AWAY; APPARENTLY BEING CAUSED BY COMPRESSOR STALLS THAT REVERSE FLOWED RAW FUEL AND OIL INTO COMPRESSOR SECTION OF ENG AND INTO AIR CONDITIONING SYS. SO WE DID THE SECONDARY CHKLIST FOR THE ENG SHUTDOWN CHKLIST. AN ILS APCH DOWN TO 500 TO 600 FT WAS EXECUTED WITH A NORMAL LNDG. POST FLT REVEALED: L ENG INLET DUCTING HAD BEEN BLOWN LOOSE FROM ENG AND FRONT OF NACELLE; L ENG CHIP-DETECTOR WAS ON FOR ENG OIL SUMP; L ENG WAS AROUND 2 QUARTS OF OIL LOWER THAN IT WAS AT PREFLT BEFORE TKOF. MECHS WERE FLOWN IN ALONG WITH MAINT INSPECTOR AND TECHNICAL REPRESENTATIVE FROM MANUFACTURER WHO INSPECTED ENG AND BELIEVE ENG BEARING FAILED ON ONE OF THE SHAFTS WHICH CAUSED COMPRESSOR STALLS; WHICH BLEW DUCTING LOOSE. ALL BANDS FOR DUCTING WAS IN PLACE AND SAFETY WIRED; BUT EDGES OF DUCTING WERE BENT LIKE THEY WERE FORCED OUT OF LOCKING BANDS. UP UNTIL THE BANGING NOISE; NO ENG INSTRUMENTS REVEALED ANY UNUSUAL OR EXCEEDANCES.