ACFT EQUIP PROB -- ACR EXPERIENCES AN UNCOMMANDED RUDDER INPUT.

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AT APPROX 1500 FT AGL ON FINAL APCH AT 170 KTS; I CALLED FOR FLAPS 15 DEGS AND GEAR DOWN WITH THE LNDG CHK. 4 OR 5 SECONDS LATER WITH FLAPS EXTENDING WE FELT A VERY HARD AND RAPID JOLT FROM THE SIDE; AS IF THE RUDDER SUDDENLY DEFLECTED TO THE R; AND BACK TO NEUTRAL AGAIN. AT THE TIME MY FEET WERE ON THE FLOOR; I QUICKLY PUT BOTH FEET ON THE PEDALS AND WAITED TO SEE IF ANOTHER JOLT OR INPUT WOULD OCCUR; HOWEVER; THE REMAINDER OF THE APCH AND LNDG WERE UNEVENTFUL. AFTER THE FLT WE DEBRIEFED THE CABIN CREW; THE 1ST FLT ATTENDANT IN FRONT DESCRIBED THE SAME SENSATIONS AS WE HAD. HOWEVER THE 3RD FLT ATTENDANT IN THE REAR OF THE ACFT ALONG WITH A DEADHEADING COMPANY PLT IN ROW 21 DESCRIBED HEARING A RUMBLING NOISE AT THE TIME IT OCCURRED. AT THE TIME OF THIS EVENT THE AIR WAS SMOOTH AND THERE WERE NO OTHER ACFT AHEAD OF US. I HAVE NOTED THIS INCIDENT IN THE ACFT LOGBOOK; HOWEVER MAINT HAS TRIED TO WRITE IT OFF AS SEVERE TURB. I HIGHLY RECOMMEND REMOVING THE PWR CTL UNIT IN THIS ACFT AND REPLACING IT. FURTHER; THE UNIT IN QUESTION SHOULD BE SUBJECTED TO TESTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A B737-300 WHEN THE RUDDER SUDDENLY DEFLECTED TO THE R AS THE FLAPS WERE EXTENDING TO 15 DEGS. THE CAPT SAID THAT THE DEFLECTION WAS ONLY A COUPLE OF DEGS; BUT THE ONSET WAS VERY RAPID AND FELT LIKE A BUMP. THE CAPT QUESTIONED THE FO; WITHOUT TELLING HIM FIRST OF THE CAPT'S IMPRESSION; AND THE FO'S DESCRIPTION WAS THE SAME ACCORDING TO THE CAPT. THE CAPT REQUESTED THAT THE RUDDER AND YAW DAMPER SYS BE REMOVED AND TESTED; BUT THE COMPANY MAINT DECIDED THAT TURB CAUSED THE 'BUMP.' SEVERAL DAYS LATER THE SAME ACFT HAD A SIMILAR EXPERIENCE ON TKOF. AT ABOUT V1 SPD THE ACFT YAWED SHARPLY AND RECOVERED. AFTER TKOF THE FLC STAYED IN THE PATTERN AND RETURNED TO LAND. THIS TIME THE PCU AND THE YAW DAMPER WERE REMOVED AND BENCH TESTED; BUT CHKED 'OK' ACCORDING TO THE MANUFACTURER'S REPRESENTATIVE THAT THE CAPT SPOKE WITH LATER. AT THE TIME OF THE FIRST INCIDENT THE FLC WAS ALSO MANUALLY FLYING THE ACFT. THE RUMBLING NOISE HEARD BY THE PLT AND THE FLT ATTENDANT IN THE FIRST EVENT WERE NOTED BY THE COMPANY MAINT PERSONNEL AND THE MANUFACTURER'S REPRESENTATIVE; BUT NOBODY SEEMS TO KNOW THE CAUSE OF THE SOUND ACCORDING TO THE RPTR.