My ap is similar and drive motor
used to "beep". The problem wasn't the drive motor, it was computer unit.
Since you saying it only does it under heavy seas I would check to see the capacity of the unit as it could be at the max of its rating. Also check/clean all electrical
connections and check the wire sizing to be sure you aren't getting too much voltage drop that only becomes a problem at high loading.
And of course it could poor sail trim!
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