What was found
DURING PRE DEPARTURE CHECKS AND JUST PRIOR TO STARTING ENGINES, HARMONIC SOUND WAS COMING FROM APU. (WINDY COOL DAY AT KSFO). WHILE COMPLETING BEFORE 'ENGINE START CHECK' APU ABRUPTLY/INSTANTANEOUSLY SHUT DOWN. RPM WENT INSTANTLY TO 05 RPM NO EXCEEDANCE OR OTHER LIMITATIONS NOTED APU SHUT DOWN ACCOMPANIED BY LIGHT SMOKE FROM TAIL PIPE ONLY NO CABIN SMOKE NOTED THROUGHOUT ALL APU OPS TO THIS POINT OF PRE-DEPARTURE WERE NORMAL. FIRE TRUCK CALLED TO STANDBY AS PRECAUTION. REMOVED & REPLACED APU ASSEMBLY IAW MM 49-10-00.