What was found
(CAN) PILOT REPORTED EXPERIENCING "MAST ROCKING" IN FLIGHT. INCREASING POWER ELIMINATED THE SYMPTOMS AS DESCRIBED BY THE PILOT: DURING CRUISE FLIGHT AT 80 KNOTS ENCOUNTERED A SLIGHT HUMP, NOSE UP/DOWN MOTION, THAT INCREASED AS COLLECTIVE LOWERED BELOW 15" MANIFOLD PRESSURE. WEIGHT JUST BELOW GROSS WEIGHT AND BALANCE WITHIN LIMITS. RAISED COLLECTIVE TO BRING MANIFOLD PRESSURE ABOVE 15" AND SLOWED TO 60 KNOTS AND MAST ROCKING ELIMINATED."