What was found
AT APPROXIMATELY 4000 FT, LOUD BANG HEARD FOLLOWED BY CALL FROM F/A REPORTING SMOKE IN CABIN AT ROW 9. INSPECTED AREA UNDER FLOORBOARDS IN CABIN AT ROW 9 AND FOUND RECIRC FAN MUFFLER NOT ATTACHED. RESECURED DUCT IAW AMM 21-20-01. NO SMOKE DETECTED, ONLY WATER VAPER FROM PACK SYSTEM.