• Verify that the tail drive pulley (red part 934 & 965) is not loose (1degree or 2degrees rotation is enough to create noise!) on the tail pulley holder (blue part 935). Disassemble the red part and look if there is not too much black dust on the blue part (due to rubbing). If the loosiness is not too big, you can clean and CA glue the part together or replace the part 934.

glue part 934 on 935 with epoxy or CA glue.

• Can be a bad pinion? (not the case on my caliber n°1)

better expalnation:

"Tail clicking noise occurs when the pulley has worn on the shaft. check out the other forum for pictures. you will need a new pulley and then have to glue the pulley to the shaft for a once and for all solution. easy way to confirm this is to hold the head, then try and move the tail rotor. if the tail rotor spins, even slightly, the shaft has begun wearing. You should only feel the resistance of the belt, and no backlash. As for twitching, this often occurs when the tail rod guides are not alligned PERFECTLY, which causes some friction on the rod. The gyro is fighting the friction by commanding more rudder input until the servo overcomes the friction and suddenly drives the tail too far, then the gyro counters this action. the result is the twitching. Both very common problems on the caliber, and should be the first thing to inspect." by vortexgen