Please check the gear mesh between the main gear and the motor pinion. If there is too much gear mesh, it would generate a lot of gear noise during flying. b. If the tail drive belt is too loose, it will bring about some noise and may cause the failure of tail control. c. Check the linkages and control system of main rotor and tail rotor to make sure every linkage is working correctly and not colliding with something else. d. Check if the motor is running out of balance.