A COMMERCIAL INST RATED PLT FLYING A PA28R FROM MLB MADE AN EMER LNDG BACK AT MLG AFTER HEARING A BANGING NOISE COMING FROM OUTSIDE THE AIRPLANE AFTER THE TKOF AND CLB.

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DURING THE TKOF PHASE OF FLT AT AN ALT OF APPROX 150-200 FT; WHILE THE GEAR WAS RETRACTING; A CONTINUOUS LOUD BANGING NOISE OCCURRED. INITIALLY I BELIEVED THAT THERE WAS A SERIOUS PROB WITH THE ENG; MAYBE A CONNECTING ROD OR STUCK VALVE. IN THE INTEREST OF SAFETY OF THE FLT; I CALLED THE TWR AND DECLARED AN INFLT EMER AND ASKED FOR IMMEDIATE CLRNC TO LAND. THE TWR CLRED ME IMMEDIATELY AND I STARTED A L TURN DOWNWIND. MY THOUGHT PROCESS WAS I WANTED TO LAND BACK INTO THE WIND DUE TO ITS DIRECTION AND SPD. AS I WAS VERY CLOSE LATERALLY TO THE RWY I WOULD HAVE LANDED DOWNWIND IF NECESSARY. AFTER A FEW MORE SECONDS; I HAD ENOUGH ALT TO TURN BACK INTO THE WIND MIDFIELD SHOULD THE ENG QUIT. DURING THE TURN I SCANNED THE ENG INSTS AND ALL WERE STILL IN THE GREEN. I INFORMED THE TWR ON DOWNWIND THAT THE ENG WAS STILL RUNNING AND I WOULD MAKE A LNDG ON RWY 9L. AS THE ENG WAS STILL RUNNING AT THIS TIME I THOUGHT THAT MAYBE SOMETHING HAD COME LOOSE ON THE GEAR/UNDERCARRIAGE; SO I EXTENDED IT AND GOT A POSITIVE 3 DOWN AND LOCKED INDICATION; BUT THE BANGING CONTINUED. I CHKED THE FLAPS AND AILERONS ON BOTH WINGS AND ALL SEEMED NORMAL. I ALSO MADE A QUICK SCAN OF THE L REAR TAIL SURFACES AND ALL SEEMED NORMAL FROM MY L SEAT POS. MY FOCUS THEN TURNED BACK TO THE ENG COMPARTMENT WHERE I THOUGHT MAYBE I HAD THROWN A BELT OR AN EXHAUST STACK HAD COME LOOSE AND WAS CAUSING THE CONTINUOUS BANGING NOISE. I MADE AN UNEVENTFUL LNDG ON RWY 9L; THE BANGING STOPPED. I TAXIED OFF THE RWY; STOPPED THE ENG AND EXITED THE AIRPLANE FOR INSPECTION. UPON EXITING THE AIRPLANE; I DETERMINED THAT THE R FRONT SEAT LAP BELT WAS THE CAUSE OF THE NOISE AS IT WAS OUTSIDE THE COCKPIT ABOUT 4-6 INCHES AND HAD BANGED INTO THE CABIN DOOR HARD ENOUGH TO SCAR THE PAINT. THE SHORT LENGTH OF THE BELT OUTSIDE CABIN EXPLAINED WHY IT BANGED ON A CONTINUOUS BASIS INTO THE CABIN DOOR AND NOT INTERMITTENTLY. I INSPECTED THE COMPLETE AIRPLANE FOR ANY OTHER DAMAGE OR CAUSE OF THE NOISE BUT COULD NOT FIND ANYTHING BUT THE LAP BELT AS THE REASON FOR THE BANGING. I CALLED THE TWR AND INFORMED THEM WHAT CAUSED THE CONTINUOUS BANGING NOISE INFLT. I GAVE MY NAME AND ADDRESS TO THE FIRE RESCUE PEOPLE AND DEPARTED IFR FOR TAMPA FOR AN UNEVENTFUL FLT. THE LESSON LEARNED FROM THIS EXPERIENCE IS THAT FROM THE PLT'S SEAT YOU CANNOT EASILY SEE IF THE R SEAT LAP BELT IS INSIDE THE CABIN WHEN THE CABIN IS SHUT IF THE LAP BELT IS NOT BUCKLED. FOR ALL FUTURE FLTS; I WILL ENSURE THAT IF THE L FRONT SEAT IS NOT OCCUPIED THE LAP BELT WILL BE FASTENED ON THE SEAT SO IT IS VISIBLE TO THE PLT AND THERE IS NO WAY THE CABIN DOOR CAN BE CLOSED WITH THE BELT OUTSIDE.