What was found
(CAN) PULLING PITCH TO TAKEOFF A BANG WAS HEARD. IMMEDIATELY THE COLLECTIVE WAS LOWERED AND THE ENGINE SET TO IDLE. THE FIRST THOUGHT WAS THAT THE SKI-BASKET HAD BEEN LEFT OPEN. NOTHING OUT OF THE NORM WAS SEEN, SO THE FLIGHT WAS CONTINUED WITH. BACK AT BASE IT WAS NOTICED THAT THE INSERT FOR A BLADE TRACKING TARGET WAS MISSING FROM 1 OF THE MAIN ROTOR BLADES. A NEW INSERT WAS INSTALLED IAW THE MM AND THE ACFT RETURNED TO SERVICE.