B737-300 CREW HAS UNEXPLAINED YAW EVENT DURING ARR TO OAK; CA.

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ACFT ORD-OAK ON DSCNT TO OAK ON THE MADWIN 3 ARR BTWN LOCKE INTXN AND SUNOL INTXN; DSNDING WITH THE 'B' AUTOPLT ON; LNAV ENGAGED; 250 KTS SELECTED IN LEVEL CHANGE; FLT ATTENDANTS AND PAX SEATED. ACFT GROSS WT 102000 LBS. AT 8000 FT MSL DSNDING TO 6000 FT MSL; THE ACFT EXPERIENCED AN UNCOMMANDED YAW EVENT LASTING APPROX 1.5 SECONDS. THE ACFT YAWED TO THE R; THE YAW WAS ACCOMPANIED BY A LOUD RUMBLE AND BANG. THE FO THOUGHT WE HIT SOMETHING. I THOUGHT WE HAD FLOWN THROUGH THE WAKE OF ANOTHER ACFT. I CHKED THE YAW DAMPER SWITCH; IT WAS ON; AND ASKED OAK APCH IF WE WERE FOLLOWING ANOTHER ACFT -- WE WERE NOT. THE 'B' AUTOPLT AND AUTOTHROTTLES REMAINED ON THROUGHOUT THE EVENT. THE AUTOPLT AND AUTOTHROTTLES WERE DISCONNECTED AROUND 6000 FT MSL; ALL ENG INDICATIONS WERE NORMAL. A VISUAL APCH WAS FLOWN TO RWY 29 AT OAKLAND. ON APPROX 1 MI FINAL THE FO (PF) THOUGHT IT FELT LIKE THE YAW DAMPER WAS NOT WORKING. I BUGGED 5 KTS EXTRA TO THE TARGET AIRSPD; WE LANDED AT OAKLAND UNEVENTFULLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID THAT THE INITIAL ROLL OF THE ACFT WAS LIKE A WAKE TURB ENCOUNTER. THERE WAS NO RPTED TFC. THERE IS RESEARCH BEING DONE TO SEE IF THERE WAS DEP TFC THAT HAD PASSED ABOVE BUT NOT UNDER CTL OF TRACON IN THE VERY RECENT PAST. WINDSHEAR WAS RPTED IN THAT GEOGRAPHIC AREA AND ALT DURING THAT DAY. YAW DAMPER WAS CHKED ON FINAL AND INDICATIONS WERE THAT IT WAS NORMAL. RPTR HAS BEEN DEBRIEFED BY COMPANY SAFETY; FLT STANDARDS; UNION SAFETY REPRESENTATIVES AND MAINT. NO MAINT PROBS DISCOVERED; HOWEVER; SEVERAL COMPONENTS WERE CHANGED. ALL INVOLVED ARE TREATING IT AS A SERIOUS INCIDENT.