MD80 FLT CREW DIVERTS DUE TO AN UNIDENTED 'THUMPING NOISE' EMANATING FROM BELOW THE CABIN SEATS. INADEQUATELY STOWED COOLER THOUGHT TO BE THE CAUSE.

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OUR FLT DEPARTED NORMALLY. FLT ATTENDANT RPTED THAT SEVERAL PAX SEATED IN THE VICINITY OF THE WING WERE RPTING HEARING REGULAR NON-MECHANICAL THUMPING OR BANGING COMING FROM BELOW THEIR SEATS. FLT ATTENDANT REMAINED IN THE AREA AND ALSO HEARD THE BANGING. HE SAID IT WAS CLRLY COMING FROM BELOW THE SEATS AND WASN'T COMING FROM ANY PAX IN THE CABIN. WE HAD NO INDICATION ON ANY COCKPIT INSTS OF ANY MALFUNCTION OR IRREGULARITY -- THE SOUND WASN'T AUDIBLE IN THE COCKPIT. ADDING TO OUR CONCERN WAS THE CLOSE PROX OF CONSTRUCTION AREAS AND POSSIBLY LESS VIGILANT RAMP MONITORING AROUND OUR DEP GATE. WE CONTACTED DISPATCH AND ADVISED THEM OF THE SIT AND OUR FEARS AND ASKED THEM TO CONTACT MIA TO SEE IF PERHAPS A RAMPER WAS MISSING OR IF THEY COULD SHED ANY LIGHT ON OUR SIT. DURING THIS TIME THE FLT ATTENDANTS ADVISED US THAT THE BANGING WAS CONTINUING. CONCERNED THAT EVEN IF MIA ACCOUNTED FOR ALL PERSONNEL WE COULD STILL HAVE A STOWAWAY; WE BEGAN A DIVERT TO ZZZ. WE ADVISED ATC OF OUR SIT; BUT WE MADE IT VERY CLR THAT WE WEREN'T DECLARING AN EMER; THAT OUR DIVERT WASN'T A MECHANICAL OR ACFT RELATED SIT. WE COORDINATED WITH ZZZ OPS AND ASKED FOR MEDICAL AND SECURITY PERSONNEL TO MEET OUR FLT. DURING OUR ARR; HOWEVER; WE DID HEAR OURSELVES REFERRED TO AS AN EMER ACFT. CLRLY; WE DID RECEIVE TFC PRIORITY AS WE LANDED ON RWY WHILE OTHER ACT WAITED TO DEPART. FEARING A STOWAWAY IN A MAIN GEAR WELL COULD FALL OUT; WE MADE NOTE OF EXTENDING THE GEAR AT ABOUT A 6 MI FINAL JUST L OF COURSE NEAR A LARGE WATER TWR. WE ALSO PLACED OUR HYD PUMPS TO LOW; HOPING THE SLOWER EXTENSION MIGHT GIVE OUR POTENTIAL NON-REVENUE RIDER SOME WARNING TO HOLD ON. LNDG WAS UNEVENTFUL. CLRING THE RWY; WE WERE MET BY ARRF. THEY INSPECTED ALL GEAR WELLS; FOUND NOTHING; AND WE TAXIED TO THE GATE. ALL BAGGAGE COMPARTMENTS WERE INSPECTED AND NOTHING SUSPICIOUS WAS DISCOVERED; EXCEPT FOR A VERY LARGE; EMPTY COOLER WHICH WAS BALANCED ATOP ANOTHER BAG. THE ZZZ PERSONNEL BELIEVE IT COULD HAVE BEEN MAKING THE BANGING NOISE AS IT TIPPED BACK AND FORTH AND BOUNCED AGAINST THE SIDEWALL. FINDING NOTHING WRONG WITH THE ACFT; WE REFUELED; DEPARTED NORMALLY AND FLEW UNEVENTFULLY.