Narrative:

Our flight departed normally. Flight attendant reported that several passenger seated in the vicinity of the wing were reporting hearing regular non-mechanical thumping or banging coming from below their seats. Flight attendant remained in the area and also heard the banging. He said it was clearly coming from below the seats and wasn't coming from any passenger in the cabin. We had no indication on any cockpit instruments of any malfunction or irregularity -- the sound wasn't audible in the cockpit. Adding to our concern was the close proximity of construction areas and possibly less vigilant ramp monitoring around our departure gate. We contacted dispatch and advised them of the situation 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 situation. During this time the flight 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 situation, but we made it very clear that we weren't declaring an emergency, that our divert wasn't a mechanical or aircraft related situation. We coordinated with ZZZ operations and asked for medical and security personnel to meet our flight. During our arrival, however, we did hear ourselves referred to as an emergency aircraft. Clearly, we did receive traffic priority as we landed on runway 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 left of course near a large water tower. We also placed our hydraulic pumps to low, hoping the slower extension might give our potential non-revenue rider some warning to hold on. Landing was uneventful. Clearing the runway, 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 aircraft, we refueled, departed normally and flew uneventfully.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.