The servo slightly moves when I switch between heading hold and rate mode, why? All trims are centred, sub-trim zero, no mixing.

This is normal functionality designed to allowing the gyro to accommodate rudder trim in rate mode.*

Spartan RC products follow the Futaba specification and use 1520uSec servo neutral. JR and Spektrum radios use 1510uSec servo neutral. This can results to a small change of the servo position when toggling between ACVS and rate modes even though the rudder trim and sub-trim are at both zero. This change is negligible and does not affect the operation and performance of your gyro.

If you wish to eliminate this small position change you can do so using a small amount of sub-trim.

• Adjust the sub-trim. Power cycle the gyro and test. Re-adjust as needed.
•  To avoid the guesswork, if you have a Flash-Link interface cable run diagnostic mode as described in [topic 24] and tweak the sub-trim until the rudder value reads 1520. 

* Important: If the rudder trim is changed the gyro must be restarted in AVCS mode so it can memorise the new rudder neutral during its power up calibration. Using rudder trim it not a recommended practice. Please consider trimming the tail mechanically instead.