What was found
AIRCRAFT WAS GROUNDED. AIRSPEED GAVE ERRONEOUS INDICATION ON CLIMBOUT ACCOMPANIED BY AIRSPEED LOW AURAL WARNING. LASTED 1 TO 2 SECONDS ONLY THEN NORMAL. OAT WAS 5 DEGREES, ENGINE ANTI ICE ON, PROBE HEATERS ON, VIS MOISTURE. 13000 FEET STANDBY AIRSPEED WAS 300 KTS. NR 2 WAS 300 KTS. PRIMARY WENT BELOW 150 KTS. FOUND MM 22-11378, LEFT ADIRU FAULT. REMOVED AND REPLACED ADIRU, IAW AMM 34-21-01.