What was found
UPON TAKEOFF WE HEARD A LOUD NOISE COMING FROM THE MAIN CABIN DOOR. SYNOPTIC PAGE SHOWED DOOR CLOSED AND VISUAL INDICATORS INSIDE SHOWED DOOR CLOSED/SECURED. AIRCRAFT PRESSURIZED NORMALLY. CONTACTED COMPANY AND RETURN TO KBNA. AFTER SHUTDOWN FOUND THE LEFT EDGE OF LARGE RUBBER SEAL AT THE TOP OF THE DOOR BENT OUTWARD. WORKED MAIN DOOR SEAL DEFLECTOR INTO ITS PROPER FORM. INSPECTED SEAL FOR ANY DAMAGE. NO DAMAGE NOTED. CLOSED MAIN DOOR AND INSPECTED DEFLECTOR FOR PROPER SEALING AROUND THE FUSELAGE. SEAL FITS PROPERLY TO THE FUSELAGE WITH NO NOTABLE GAPS. NO DEFECTS NOTED AT THIS TIME. IAW AMM 52-10-24.