I had this problem, and found a matching thread on this forum, which, however did not include my solution. Thus I would like to add to this, for the next person looking for help.
The problem was simple: The fan was braked mechanically by a power chord that had shifted.
Unfortunately, this is a serious design weakness in NUC9's. Space around the graphicscard is extremely limited, and there is virtually no room for the unused power connectors and their cables. Yet, for some reason, only one half of only one fan is protected by a grille, instead of both fans being fully covered.
Eventually though, after about an hour, I managed to tuck the unused power connectors away.
Hope this helps anyone with a similar problem.
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I hope you are doing fine.
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Intel Customer Support Technician.