What was found
CREW REPORTED LEFT HAND ENGINE VIBRATION AND SLIGHT SURGING, ALL ENGINE INDICATIONS REPORTED NORMAL. CREW PERFORMED MAG CHECK (NORMAL) AND TURNED THE LEFT AUXILIARY PUMP OFF, THEN TO HIGH AND STILL HAD THE VIBRATION, SLIGHT SURGING AND DROPPING RPM. CREW DECIDED TO FEATHER THE ENGINE AND SHUT IT DOWN. COMPANY MAINTENANCE INSPECTED AND FOUND THE INTERNAL CHECK VALVE ON THE LEFT SIDE AUXILIARY PUMP WAS STUCK. THIS PUMP WAS REMOVED AND REPLACED WITH AN OVERHAULED UNIT. ALL FUNCTIONAL CHECKS NORMAL AFTER REPLACEMENT.