AN MD88 DURING CLBOUT AT 11000 FT DIVERTED DUE TO CABIN ATTENDANT'S RPT OF LOUD BANGS AND RUMBLES FROM THE L ENG ON TKOF. INSPECTION FOUND ENG OK FOR SVC.

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I WAS THE FO AND PF ON A 4-HR PLANNED MD88 FLT TO A WEST COAST DEST. FOLLOWING AN UNEVENTFUL TAXI OUT AND DEP; I INITIATED A R TURN TO THE ASSIGNED DEP HDG AND FOLLOWED OUR NOISE ABATEMENT CLB PROFILE. WE REQUESTED AND RECEIVED A FURTHER TURN TO AVOID AN AREA OF LEVEL 3 AND 4 TSTMS THAT WERE APCHING THE FIELD. PASSING 8500 FT; WHILE STILL IN STERILE OP; THE CAPT TOOK A CALL ON THE COCKPIT PHONE FROM THE AFT FLT ATTENDANT WHO SAID THAT LOUD BANGS AND VIBRATIONS WERE COMING FROM THE L ENG. ALL COCKPIT INDICATIONS WERE NORMAL AND NO VIBRATIONS COULD BE FELT ON THE THROTTLES. WE LEVELED OFF AT 11000 FT; THE CAPT ASSUMED ACFT CTL AND SENT ME BACK TO INVESTIGATE. AFTER LISTENING FROM DIFFERENT POINTS IN THE BACK OF THE JET; FEELING FOR VIBRATION AND VISUALLY INSPECTING THE L ENG FROM THE AFT GALLEY WINDOW; NOTHING UNUSUAL WAS FOUND. I INTERVIEWED THE 3 AFT FLT ATTENDANTS WHO DESCRIBED THAT LOUD THUMPING; GRINDING AND POPPING SOUNDS HAD ACCOMPANIED FLOOR VIBRATIONS JUST INSTANTS AFTER ROTATION AND HAD BEEN LOUD ENOUGH TO CAUSE PAX IN THE AREA TO TURN AROUND AND LOOK AT THEM FOR REASSURANCE. AFTER RETURNING TO THE COCKPIT AND BRIEFING THE CAPT; WE CONCLUDED THAT A RETURN TO OUR DEP ARPT WAS WARRANTED. THIS DECISION TO RETURN WAS BASED SOLELY ON THE INFO PROVIDED FROM THE FLT ATTENDANTS; SINCE THE JET WAS PERFECTLY NORMAL FROM OUR PERSPECTIVE. WE CONSIDERED THE FACT THAT WE HAD SENIOR FLT ATTENDANTS ON BOARD WHO WERE INTIMATELY FAMILIAR WITH NORMAL ENG AND SYS SOUNDS; ALTHOUGH; I MUST ADMIT THAT I WAS SOMEWHAT LEARY AS I HEADED BACK FOR MY INSPECTION. PRIOR TO INTERVIEWING THE CREW; I THOUGHT; 'JUST ANOTHER PARANOID FLT ATTENDANT THAT JUMPS AT EVERY LITTLE SOUND.' FOR ME; IT WAS THE FACT THAT ALL 3 FLT ATTENDANTS SHOWED CONCERN AND THAT THE PAX HAD BEEN DISTRESSED; THAT CONVINCED ME WE HAD TO BRING THE BIRD BACK TO THE NEST. HAVING PERFORMED ALL THE REQUISITE NORMAL CHKLISTS; WE REQUESTED AN APCH TO THE LONGEST RWY WITH THE 'EQUIP STANDING BY.' ALTHOUGH NO EMER WAS DECLARED; APCH CTL ASKED AND I RELAYED SOULS ON BOARD AND FUEL REMAINING. AT THIS POINT; IT WAS A RACE TO BEAT THE TSTMS APCHING THE AIRFIELD. WE KEPT OUR SPD UP UNTIL SHORT FINAL. WE ALSO CHOSE TO USE MEDIUM AUTO BRAKES AND FULL FLAPS FOR LNDG SINCE WE WERE OVERWT AND THE RWY WAS WET. WE LANDED UNEVENTFULLY 58 MINS AFTER TKOF. ON ROLLOUTS; BOTH ENGS WERE STILL RUNNING FINE. NONETHELESS; WE SHUT DOWN THE SUSPECT ENG AND TAXIED BACK UNDER ESCORT BY EMER VEHICLES. WE TAXIED TO THE GATE AND DEPLANED THE PAX VIA THE JETWAY. THE ACFT WAS TAKEN OUT AND IS CURRENTLY UNDER INSPECTION. UPON DEBRIEFING WITH THE FIRE CAPT; HE INFORMED US THAT HE HEARD A LOUD BACKFIRE AS WE PASSED HIM ON LNDG ROLLOUT; EVEN THOUGH OUR FLT ATTENDANTS DID NOT HEAR ANYTHING UNUSUAL AT THAT TIME. FOLLOW UP: SUBSEQUENT INSPECTION REVEALED NO ENG DISCREPANCIES. THE ACFT HAS SINCE RETURNED TO SVC UNEVENTFULLY.