What was found
(CAN) DURING A ROUTINE PATROL FLIGHT THE PILOT EXPERIENCED A VIBRATION IN THE AREA OF THE REAR ENGINE. HE ELECTED TO RETURN TO THE AIRPORT AND MADE AN UNSCHEDULED LANDING WITHOUT INCIDENT. UPON INSPECTION BY THE MX CREW, IT WAS DISCOVERED THAT THE FWD EDGE OF THE INNER & OUTER SECTIONS OF THE AIR COOLING DUCT HAD PARTIALLY SEPARATED FROM THE NOSE CAP. THIS ALLOWED ROM AIR TO FLOW BETWEEN THE INNER & OUTER SECTIONS AND CAUSED THE INNER DUCT TO VIBRATE AND SUBSEQUENTLY CRACK & BREAK.