A B737-300 AT 13000 FT HAD THE STAB TRIM WHEEL START TO 'CHATTER' THEN BEGAN 'FREE-WHEELING.' CREW DECLARED AN EMER; AND LANDED UNEVENTFULLY; BUT WAS UNABLE TO FIND A CORRESPONDING ABNORMAL PROCEDURE THAT MATCHED THE ACFT CONFIGURATION.

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DSNDING FROM 13000 FT MSL TO 10000 FT MSL SLOWING FROM 300 TO 250 KTS. HORIZONTAL STAB TRIM STARTED TO TRIM FOR SLOWER AIRSPD. AS WE APCHED 250 KTS; NOTICED STAB TRIM WHEEL START TO 'CHATTER' AND NOT ALLOW ANY MORE TRIM MOVEMENT. A FEW SECONDS LATER; TRIM WHEEL SEEM TO BREAK FREE AND FREE-WHEEL ON ITS OWN. I DISCONNECTED AUTOPLT 'A' AND AUTOTHROTTLE AND CALLED FOR THE UNSCHEDULED TRIM QRC. FO ACCOMPLISHED CHKLIST; HOWEVER; IT DIDN'T REALLY ADDRESS OUR PROB OF A FREE-WHEELING; DISCONNECTED STAB TRIM WHEEL. I TOOK OVER THE RADIOS AND HAND FLEW THE ACFT WHILE FO LOOKED FOR A MORE APPROPRIATE IRREGULAR. I ALSO ASKED HIM TO CONTACT DISPATCH AND MAINT TO ASSIST IN FINDING PROPER IRREGULAR TO USE. APCH GAVE US A BOX PATTERN APPROX 30 MILES W OF FIELD AT 7000 FT/250 KTS. EVENTUALLY I TURNED AUTOPLT 'A' BACK ON; ONCE I HAD A BETTER GRASP ON SITUATION. FO HAD A DIFFICULT TIME HEARING MAINT ON FREQ AND EVENTUALLY SWITCHED TO ANOTHER FREQ. MAINT KEPT SENDING ACARS TO ACCOMPLISH IRREGULAR PROC; BUT DIDN'T SPECIFY WHICH ONE TO DO. WE HAD A HARD TIME DETERMINING WHICH IRREGULAR PROC TO USE AND THOUGHT THAT MAINT COULD BETTER DIRECT US; BUT THAT WASN'T THE CASE. MAINT DID HAVE US MAKE SURE THAT STAB TRIM ACT P6-2/B11 CB WAS NOT POPPED AND DIRECTED US TO RESET THE HORIZONTAL STAB CB; BUT DIDN'T TELL US WHERE IT WAS LOCATED. FO EVENTUALLY FOUND AND RESET THE CB AT P18-0 WHEN WE WERE INBOUND FOR APCH TO NO AVAIL. AROUND 4000 LBS OF FUEL REMAINING I DECLARED AN EMER AND TOLD ATC AND FO THAT WE WERE GOING TO LAND IN 10 MINUTES. MAINT WAS STILL GIVING CONFUSING INFO TO FO SO I TOLD HIM TO CURTAIL CONVERSATION SO WE COULD BRIEF THE APCH. WE ELECTED TO DO THE JAMMED STAB TRIM IRREGULAR AS THIS PROC WAS CLOSE TO; BUT NOT EXACTLY THE SITUATION WE HAD. EARLIER I INFORMED FLT ATTENDANTS OF OUR PROB AND TOLD THEM IT WAS A CABIN ADVISORY AND THAT WE WOULD BE ON THE GND IN ABOUT 15 MINUTES. I ALSO INFORMED THE PAX OF OUR SITUATION OVER THE PA. WE LANDED IN VMC CONDITIONS WITH FLAPS 15 IN ACCORDANCE WITH IRREGULAR PROC. LNDG WAS UNEVENTFUL.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THAT AT THE GATE; MAINT FOUND THE STAB TRIM WHEEL DRIVE CHAIN SEPARATED. THE CHAIN HAD REPORTEDLY SEPARATED AT THE MASTER LINK CONNECTION BUT THE MASTER LINK AND THE TWO COTTER PINS COULD NOT BE FOUND. SPECULATION WAS THE LINK WAS NEVER INSTALLED AT THE LAST HVY MAINT CHK.