Have a look in task manager to see what/if any processes have high cpu use.
Could be an App./software set to run on startup.
Could be an incorrect bios setting.
What make/model motherboard ?
And what monitoring App. is showing the high cpu clock/fan speed ?
I am not familiar with that motherboard but my Asrock z370 has a sort of multi core enhancement setting in the bios and if that is enabled they will run at max clocks.
I don't know what Asus calls it but it could be something like that