A B737-400 IN CRUISE AND DSCNT EXPERIENCED RUDDER PULSATIONS EVERY 3-5 SECONDS AND WAS OBSERVED ON THE YAW DAMPER RUDDER INDICATOR.

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APPROX 1 HR 15 MINS OUT OF LAX AT FL350 AFTER GOING THROUGH LIGHT TURB; OUR JUMP SEAT RIDER ASKED IF WE FELT A YAW MOVEMENT. YES; IT WAS NOTICEABLE. CAPT THEN ASKED ME IF THIS WAS NORMAL ON THE B737. I TOLD HIM IT WAS NOT. WE DECIDED TO TRY AUTOPLT 'B'. THE PULSATION EVERY 3-5 SECONDS CONTINUED AT CRUISE AND ON DSCNT. ONCE THE FLAPS WERE EXTENDED BEYOND 10 DEGS; IT THEN TOOK 2-3 UNITS OF AILERON TRIM TO FLY STRAIGHT AND LEVEL. WE CONTINUED THE VISUAL APCH AND LANDED IN LAX WITH NO FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE YAW INPUTS WERE NOTICEABLE EVERY 3-5 SECONDS AND THE FIRST REACTION WAS TO TRY SWITCHING TO ANOTHER AUTOPLT CHANNEL BUT NO CHANGE OCCURRED; SO THE AUTOPLT WAS SWITCHED OFF. RPTR ADVISED IN DSCNT WITH 10 DEGS FLAPS SELECTED HAD TO ADD 3-5 UNITS OF AILERON TRIM FOR STRAIGHT AND LEVEL FLT. THE RPTR SAID THE RPTS IN THE LOG WERE WRITTEN SO THE ACFT HAD TO BE WORKED ON ON THE OVERNIGHT LAYOVER AND MAINT RPTED REPLACING THE YAW DAMPER COUPLER TO CORRECT THE RUDDER PULSATIONS AND ADJUSTED 1 FLAP JACKSCREW FOR TRIM CORRECTION. RPTR STATES THE ACFT TEST FLEW OK. THE RPTR RELATED THAT MAINT ADVISED THE ACFT MANUFACTURER OF THE RUDDER RPT. THE RPTR RPTS FLYING THE ACFT THE NEXT DAY BACK TO ZZZ AND ALL SYS WERE OK; BUT ON ARR AT ZZZ THE ACFT MANUFACTURER ADVISED REPLACING THE RUDDER PWR UNIT. THE RPTR STATES THIS WAS DISTURBING BECAUSE THE UNIT SHOULD HAVE BEEN REPLACED ON THE OVERNIGHT LAYOVER WHEN THE OTHER ITEMS WERE CORRECTED. CALLBACK CONVERSATION WITH RPTR ACN 377167 REVEALED THE FOLLOWING INFO: RPTR STATED THE MAINT ACTION TO CORRECT THE RPTS WERE AVAILABLE BUT DID KNOW THAT 28-29 INPUTS WERE RECORDED ON THE FLT DATA RECORDER.