B737-300 FLT CREW HAS A GPWS DURING FINAL APCH; WHILE TWR ISSUES A SIDESTEP TO PARALLEL RWY AS A RESULT FLT CREW MISSES CLRNC.

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FROM JUST INSIDE BEARR INTXN (ON BEARR 4 STAR) WITH APCH; WE ARE TOLD TO EXPECT VECTORS ILS RWY 16L. A COUPLE OF SLOW DOWN; 'DIRECT IRONN; INTERCEPT LOC RWY 16L...CROSS IRONN AT 11000 FT; CLRED FOR THE ILS RWY 16L; MAINTAIN 170 KTS TO YYIP.' CONTACT TWR 132.65. TWR ACKNOWLEDGED THE INITIAL CALL AND DIRECTED US TO CALL 'FIELD IN SIGHT.' PF WAS FULLY CONFIGURED FOR THE APCH AND STABILIZED ON THE ILS RWY 16L. JUST INSIDE THE FAF; SIMULTANEOUSLY WE GOT A 'TERRAIN; TERRAIN; PULL UP; PULL Up' FROM OUR GPWS AND TWR SWITCHING US TO 118.3; AS I WAS CALLING THE FIELD IN SIGHT. PF IMMEDIATELY INITIATED A CLB WHILE WE WERE TRYING TO UNDERSTAND WHAT COULD HAVE TRIGGERED THE GPWS AND AT THE SAME TIME I RESPONDED TO THE TWR FREQ CHANGE WITH A SHORT 'ROGER' AS I WAS BUSY WITH THE OTHER MATTER. GPWS STOPPED ITS WARNING AND THE PF WAS IN A GOOD POS TO CONTINUE THE APCH TO RWY 16L. AS WE WERE ALREADY ON TWR; I COULD NOT UNDERSTAND THE TWR FREQ CHANGE AND HAD NOTICED ON MY CHART THAT IT WAS THE FREQ FOR RWY 17. ON MY CALL TO 118.3 I STATED; 'ON FINAL RWY 16L; I DON'T KNOW WHAT I'M DOING WITH YOU.' TWR *118.3) SAID WE WERE SUPPOSED TO LINE UP WITH RWY 17. I DISPUTED RECEIVING THAT INSTRUCTION AND HE CLRED US TO LAND ON RWY 16L. THERE WAS NEVER A CONFLICT WITH ANY OTHER SHIP IN THE AIR OR ON THE GND. AS WE EXITED THE RWY; AGAIN WE WERE TOLD THAT WE SHOULD HAVE LINED UP WITH RWY 17 AND AS I WAS TRYING TO EXPLAIN WE NEVER GOT THAT MESSAGE; WE WERE TOLD TO CONTACT TWR ON THE PHONE. REPEATED THE SAME VERSION OF THE FACTS WITH THE TWR ON THE PHONE AND LATER THE TAPE REVEALED THAT WE WERE WRONG AND THE ONLY EXPLANATION FOR THE ERROR WAS THAT GPWS WAILING IN THE BACKGND AT THE CRITICAL TIME AND COVERING PART OF THE TWR XMISSION.