What was found
(CAN) AFTER A NORMAL TAKEOFF AND CLIMB PHASES, THE CREW NOTICED DURING CRUISE THAT THE ENGINE PROPELLER OSCILLATED WITH A GRADUAL INCREASE ON PROPELLER SPEED. THE CAPTAIN DECIDED TO STABILIZE ACFT AT THAT FLIGHT LEVEL TO MONITOR THE PROPELLER OSCILLATION. HE THEN TRIED TO REDUCE PROPELLER SPEED BACK TO 40 PERCENT BUT PROPELLER OSCILLATION REMAINED. DUE TO THIS, THE CAPTAIN DECIDED TO SHUTDOWN THE ENGINE AND RETURN TO ORIGIN UNEVENTFULLY.