It is not unusual for a mast at rest to develop a harmonic motion from wind blowing at the right speed from the right direction. It is not a problem, other than providing an irritating distraction. You shouldn't mess with your standing rig to try to remove it, as you want your rig set up right for its primary purpose. you may find adding/removing backstay or babystay tension tohave an effect, or try adding some tension to the middle of the mast by tensioning a main halyard forward under a spreader root, anything that disrupts the harmonic balance.
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