A B737-300 IN CLB CREW NOTED AIRFRAME VIBRATION AT FL310 CABIN ATTENDANTS NOTED SEVERE VIBRATION OVERWING AREA. FLT RETURNED TO DEP ARPT.

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ON CLB FROM ZZZ THROUGH FL180; THE CAPT AND I NOTICED A LOUD HUM. I CALLED THE FLT ATTENDANTS AND THEY SAID THEY COULD HEAR THE NOISE ALSO AND THERE WAS MODERATE VIBRATION ASSOCIATED WITH IT. THE CAPT SAID THERE WAS A WRITE-UP PREVIOUSLY DEALING WITH ACFT VIBRATION. THE ACFT HAD AN MEL 21-4 C1 DEALING WITH THAT WRITE-UP. I THEN CALLED MAINT CTL AND TOLD THEM ABOUT OUR PROB. THEY LOOKED UP THE ACFT HISTORY AND TOLD US ABOUT THE MEL DEALING WITH THE PACK TURBO FAN. THE CAPT AND I DIDN'T FEEL THAT WAS THE VIBRATION WE WERE HEARING AND FEELING. THE FLT ATTENDANT THEN CALLED US AND SAID THE VIBRATION WAS BAD OVERWING AND THAT A COUPLE OF PAX HAD SMELLED BURNING RUBBER AND THAT SEVERAL PAX WERE GETTING EXTREMELY NERVOUS AND SCARED. I WENT TO THE CABIN TO CHK ON THE SMELL AND VIBRATION. I DIDN'T NOTICE ANY SMELL BUT THE VIBRATION WAS MODERATE OVERWING. AT THIS POINT THE CAPT DECIDED TO RETURN TO ZZZ. ON DSCNT INTO ZZZ; THE VIBRATION STOPPED AS WE CAME THROUGH FL180. NO EMER WAS DECLARED; THE LNDG WAS OVERWT (117100 LBS); BUT EVERYTHING ELSE WAS NORMAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID NO INFO ON THE CAUSE OF THE AIRFRAME VIBRATION WAS BEEN RELEASED.