Hi Oliver , this is normal.. When machine boots up , FAN speed gone up for RPM test , then goes down to normal.
So feel free to use that FAN, its not an issue .
We would like to inform you that the behavior you are experiencing is completely expected and normal. The reason why the fan suddenly goes up and then back to normal is because the Management Engine is taking control of the system during POST. There is no reason to be concerned about this matter.