What was found
(CAN) PILOT REPORTED LOUD NOISE/VIOLENT VIBRATION ON TAKEOFF, WHILE AIRBORNE. AFTER DISCUSSING WITH PILOT, FOUND OUT THAT FLIGHT CONTROLS WAS NOT THE ISSUE, MOST PROBABLY ENGINE RELATED. AFTER TAKEOFF ENGINE WAS VERY ROUGH AND LOUD. COMPLETED CIRCUIT, WHEN TOUCHDOWN THE VIBRATION STOPPED. FOUND ALTERNATE AIR VALVE BODY SEPARATED AT CONTROL CABLE ATTACHMENT WHICH WOULD THEN ALLOW BODY TO ROTATE FREELY CAUSING THE ALTERNATE AIR TO BE APPLIED UNVOLUNTARILY. NEW VALVE BODY INSTALLED. MAINTENANCE GROUND RUN AND HIGH SPEED TAXI REVEALED NO FAULT AFTER REPAIRS, ACFT RELEASED CONDITIONAL TO SATISFACTORY TEST FLIGHT.