With most systems, upon being powered on, all of the fans in the system will spin at maximum speed until POST is completed. This is for two reasons:

Most fans prefer to start on their highest setting. If they start on their lowest setting, it may take a while for them to spin up.

The other reason being that it is safer to have the default set to have the fans spin at full speed. Should something fail, your components would be safer from overheating.