AN ACR B737 FLC EXPERIENCED SEVERAL YAW DAMPER DISPLACEMENTS AND ONE OF THEM WAS VIOLENT ENOUGH TO CAUSE THE AIRFRAME TO 'POP' WHILE IN CRUISE.

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ON MAR/MON/98; WE WERE ON A SCHEDULED FAR 121 FLT FROM SPOKANE TO SEATTLE; WA. THE TKOF WAS NORMAL AND ON CLBOUT DURING A R TURN THE CAPT NOTICED THAT THE YAW DAMPER INDICATION WAS SHOWING FULL SCALE R DEFLECTION WITH MY FEET FLAT ON THE FLOOR. DURING THE CLB I ADJUSTED THE SLIP/SKID INDICATOR TO POS THE BALL IN THE CTR OF THE INDICATOR. WE HAD BEEN AT CRUISE FOR PROBABLY 10 MINS WHEN A POP WAS HEARD AND THE ACFT YAWED TO THE R AND THE AUTOPLT CORRECTED IT BY PUTTING IN 10-15 DEGS OF BANK. THE ACFT OSCILLATED A FEW TIMES; EACH TIME THE DEG OF OSCILLATION GOT SMALLER. I GRABBED HOLD OF THE YOKE AND PUT MY FEET ON THE RUDDER PEDALS AS SOON AS THE POP WAS HEARD AND TRIED TO HELP THE AUTOPLT DAMPEN OUT THE OSCILLATIONS. THE CAPT TURNED THE YAW DAMPER OFF AND THEN RAN THE UNCOMMANDED YAW CHKLIST. I NEVER CALLED FOR THE CHKLIST AS HE HAD THE CHKLIST OUT BEFORE I COULD SAY LET'S RUN THE CHKLIST. WE DID THE CHKLIST AND I TURNED THE AUTOPLT OFF PER THE CHKLIST AND STARTED TO DSND. WE HAD BEEN GIVEN DISCRETION TO FL240. DURING OUR DSCNT THE CAPT SAID TO REENGAGE THE AUTOPLT AS HE THOUGHT THE YAW DAMPER WAS THE CULPRIT. WE CONTINUED THE FLT MAKING SURE THE PAX AND THE FLT ATTENDANTS WERE NOT HURT. THE C FLT ATTENDANT SAID HE FELT THE ACFT OSCILLATING BACK AND FORTH DURING THE FLT WITH THE OSCILLATIONS GETTING A LITTLE MORE NOTICEABLE EACH TIME. WE LANDED WITHOUT INCIDENT IN SEATTLE AND PULLED THE FLT DATA RECORDER AND THE COCKPIT VOICE RECORDER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO SAID THAT HE WAS FLYING THE B737-300 USING THE CTL WHEEL STEERING MODE OF THE AUTOPLT WHEN THE CAPT STATED THAT THE YAW INDICATOR SHOWED THE RUDDER DISPLACED FULL (YAW DAMPER LIMIT) TO THE R. HE SAID; THAT HE HAD NOT NOTICED ANY ABNORMAL FEEL OR ACFT MOTION TO THAT TIME IN THE FLT AND SINCE THE ACFT CONTINUED TO FLY WITHOUT ANY UNUSUAL MOTIONS THEY CONTINUED THE CLB. HE SAID; THAT WHEN HE LOOKED AT THE YAW INDICATOR IT WAS CTRED. DURING CRUISE; WHEN THE ACFT YAWED TO THE R THE WHOLE AIRFRAME POPPED ACCORDING TO THE RPTR. AGAIN; HE PLACED HIS HANDS AND FEET ON THE CTLS; BUT HE LEFT THE ACFT ON AUTOPLT SINCE IT WAS CORRECTING PROPERLY. HE SAID THAT HE DID NOT FEEL ANY GREAT DISPLACEMENT OF THE CTLS OTHER THAN THE FEW AUTOPLT CORRECTIVE INPUTS. THE CAPT COMPLETED THE UNCOMMANDED YAW CHKLIST AND ENTERED THE EVENT IN THE MAINT LOG. UPON ARR IN SEA THE CREW BRIEFED CONTRACT MAINT PERSONNEL AND THEN HAD TO DEPART FOR THE NEXT DEST IN ANOTHER ACFT. HE HAS NOT HEARD OF ANY FINDING; BUT HE KNOWS THAT HIS REGULAR COMPANY MAINT OP SENT A TEAM TO SEA TO INSPECT THE ACFT. SUPPLEMENTAL INFO FROM ACN 396530: WHILE IN CRUISE AT FL350 AT MACH .755 THE RUDDER KICKED HARD R. THE ACFT TAIL SWUNG VIOLENTLY TO THE L AND THE ACFT ROLLED 12-15 DEGS TO THE R. THE AUTOPLT THEN COUNTERED THE ROLL AND OVERCORRECTED BACK TO ABOUT 5-7 DEGS L WING DOWN. THE YAW DAMPER INDICATOR SHOWED FULL DEFLECTION TO THE R. THE YAW DAMPER WAS TURNED OFF AND THE ACFT STABILIZED. QRH ITEMS FOR UNCOMMANDED ROLL WERE ACCOMPLISHED DURING THE DSCNT.