ON N DOWNWIND VECTOR TO OAK ILS RWY 11 BY NCT APCH CTL; A B737-300 FLT CREW READ BACK INCORRECT DSCNT ALT RESULTING IN A GPWS ALERT CLB.

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WE WERE BEING VECTORED TO RWY 11 AT OAK. WE WERE AT 4000 FT ON DOWNWIND TO THE ILS TO RWY 11 NE OF THE FIELD. WE THOUGHT THE CTLR SAID DSND TO 2100 FT. WE WERE ABOUT ABEAM THE OM FOR THE APCH. THE FO READ BACK 2100 FT AND I STARTED TO DSND TO 2100 FT. THE MINIMUM ALT FOR THAT AREA IS ABOUT 3800 FT; BUT WE WERE ABOUT TO TURN TOWARD THE FAF AND WE WERE UNDER HIS CTL. AT 2100 FT WE RECEIVED A 'TERRAIN; TERRAIN; PULL UP.' I IMMEDIATELY EXECUTED A CLB AND WAS CLBING THROUGH 3000 FT WHEN THE CTLR ASKED US TO CONFIRM THAT WE WERE AT '3100 FT.' WE TOLD HIM THAT WE WERE LEVELING OFF FROM A CLB FROM 2100 FT AND WE UNDERSTOOD THAT HE HAD GIVEN US A DSCNT TO 2100 FT AND HAD READ THAT BACK TO HIM. HE TOLD US TO TURN AND DSND TO 2000 FT UNTIL LOC INTERCEPT AND THEN INTERCEPT THE GS AT 1800 FT. APCH AND LNDG WAS UNEVENTFUL. THIS CTLR TALKED WITH A SPEECH IMPEDIMENT AND WHEN HE SPOKE FAST IT WAS DIFFICULT TO UNDERSTAND HIM. WHY HE DIDN'T CORRECT US WHEN WE READ BACK THE WRONG ALT; I DON'T KNOW. HE WAS BUSY. AFTER WE HAD OUR PROB; HE DID SLOW HIS SPEECH DOWN AND IT WAS EASIER TO UNDERSTAND. I SHOULD HAVE QUESTIONED HIM FURTHER ON THE 2100 FT INSTRUCTION BECAUSE IT WAS AN ODD ALT WITH THAT MINIMUM SECTOR ALT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CAPT SAID THAT HE WAITED A BIT TOO LONG TO CHALLENGE THE DSCNT CLRNC; ESPECIALLY KNOWING THAT HE WAS HEADING FOR HIGHER TERRAIN. BY THE TIME IT BOTHERED HIM SUFFICIENTLY TO QUESTION ATC; THE RPTR ADVISED THE GPWS WAS ALARMING. THE OTHER PRINCIPLE PROB WAS THE CTLR'S SPEECH RATE. IT WAS VERY FAST AND APPEARED TO HAVE AN IMPEDIMENT. AFTER THE CTLR QUESTIONED THE FLT CREW'S DSCNT ALT; AND THE CAPT'S CHALLENGE RESPONSE; THE CTLR'S SPEECH RATE SLOWED DOWN SUFFICIENTLY TO BE UNDERSTOOD.