FLC OF B727-300 HAS GPWS ACTIVATE ON APCH IN NIGHT IFR CONDITIONS.

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ACFT LEVEL AT 3100 FT. OUR ACFT WAS BEING VECTORED ON A BASE TO FINAL APCH AT OAK. THE ASSIGNED HDG WAS 260 DEGS AND THE ASSIGNED ALT WAS 3100 FT. OUR POS WAS APPROX 11 MI NNE OF THE OAK ARPT. WE RECEIVED AN AURAL TERRAIN WARNING. WE IMMEDIATELY ADDED MAX PWR; CLBED; NOTIFIED ATC AND CHKED TCASII FOR POSSIBLE CONFLICTS. THE ALTIMETERS WERE CORRECTLY SET. ATC RESPONDED THAT THE ALT WAS CORRECT FOR OUR FLT AREA. FLT CONDITIONS WERE IMC. AN UNEVENTFUL APCH AND LNDG FOLLOWED. WE CHKED ALL PARAMETERS AND APCH CHARTS TO DETERMINE THE REASON FOR THE WARNING. WE WERE ON THE CORRECT ASSIGNED HDG AND ALT WITH THE ALTIMETERS CORRECTLY SET. IT APPEARS THAT WE SHOULD HAVE BEEN CLR OF TERRAIN AT THIS POINT. THE REASON FOR THE WARNING REMAINS UNCERTAIN AND FURTHER STUDY IS BEING CONDUCTED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THEY WERE FLYING A B727- 300. HE SPOKE WITH THE CHIEF PLT AND WAS INFORMED THAT THE COMPANY HAS HAD ABOUT 6 OR 7 SUCH INCIDENTS. HIS COMPANY IS TRYING TO DETERMINE IF SOME CHANGE MIGHT BE MADE IN THE MVA IN THIS AREA. IN THIS INSTANCE; THE FREQ WAS VERY CONGESTED AND THE FLT WAS IN AND OUT OF WX. THEY WERE NOT ABLE TO INFORM CTLR THAT THEY HAD CLBED AND WERE CONCERNED ABOUT THE MANY OTHER ACFT IN THE AREA. RPTR WAS EXTREMELY GRATEFUL FOR TCASII SINCE HE COULD KEEP TRACK OF PROX TO OTHER ACFT. THE FLC RESPONSE WAS IMMEDIATE AND AGGRESSIVE AND MANY PAX WERE DISTURBED BY THE ABRUPT ACTION. RPTR WISHED HE HAD HAD TIME TO MAKE A PA AS MANY PAX STOPPED TO ASK WHAT HAPPENED AS THEY DISEMBARKED.