A B733 CREW; DSNDING INTO GEG; RECEIVED A GPWS WARNING; 'TOO LOW TERRAIN' WITH A 'PULL UP' COMMAND.

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WE WERE BEING VECTORED FOR AN ILS APCH AT SPOKANE. DURING THE TURN FROM BASE TO DOGLEG WHILE DSNDING FROM 4700 FT TO 4500 FT ASSIGNED; THE GPWS SOUNDED A TERRAIN WARNING FOLLOWED BY A PULL-UP VERBAL COMMAND AND LIGHT. WE IMMEDIATELY EXECUTED A MAX PWR CLB TO MINIMUM SAFE SECTOR ALT AND ADVISED ATC. THERE WAS NO NEAR-BY TFC ON 'TSAS' OR RPTED BY ATC. ONCE WE CONFIRMED WITH ATC WE WERE CLR OF THE TERRAIN; WE RESUMED THE APCH. DURING THE ORIGINAL VECTOR FOR THE ILS; OUR RADAR SHOWED ONLY LIGHT RAIN IN ALL DIRECTIONS. VISUALLY; THE PRECIP ALTERNATED BTWN SNOW AND RAIN. NO ICE WAS ACCUMULATING ON THE WINDOWS. AFTER LNDG; I DISCUSSED THE EVENT WITH THE AIR TFC CTLRS. THEIR RADAR SHOWED LEVEL 1 PRECIP THROUGHOUT THAT AREA EXCEPT FOR A 3 MI DIAMETER AREA OF LEVEL 2 PRECIP TO OUR R FRONT. AS WE MADE THE L TURN TO DOGLEG; THIS NEAR-BY LOCALIZED HEAVIER AREA OF PRECIP AT OR NEAR THE FREEZING LEVEL MAY HAVE BEEN RECOGNIZED AS A SOLID OBJECT BY THE GPWS AND CAUSED THE WARNING. SUPPLEMENTAL INFO FROM ACN 530747: APCH GAVE US VECTORS AND DSCNT TO THE ILS RWY 21. THE ILS AND LNDG WERE UNEVENTFUL. POINTS OF INTEREST: 1) THE 'WHOOP; WHOOP' SOUND WAS NEVER HEARD; JUST 'TERRAIN; TERRAIN; PULL UP; PULL UP.' 2) NEITHER THE CAPT NOR FO SAW THE RADAR ALTIMETER TOP SCALE PRESET. 3) ON THE GND; APCH STATED THAT THERE WAS NOT TERRAIN AT THAT ALT AND POS TO MERIT THE WARNING AND THEY FELT A LEVEL 2 RAIN SHOWER AT OUR POS HAD TRIGGERED THE ALERT. 4) NO ACFT MALFUNCTIONS OR ANOMALIES NOTED. 5) NO PAX ON BOARD -- FERRY FLT.