I have noticed this as wel, with a Librem13V2, on PureOS. The cpu fan goes to maximum and will not stop.
There is no heightened cpu usage or a high temperature. I suspect it is a corner case in the thermal throttling in the firmware.
Allow the laptop to go into sleep mode (I don’t know how long that is, guessing 10 minutes), the screen goes black and the wifi led turns off.
When you touch the mousepad or keyboard again, the screens turns on, showing the lock screen. If I type my password in quickly, I get the result as above; the cpu fan starts spinning at maximum.
Strangely, if I first mistype my password or just take my sweet time filling in my password, the cpu fan does not spin at maximum.
As suggested above by others, the temporary fix is to press the power button for 2 seconds, which causes the laptop to go back to sleep (since that is the default option in PureOS - Settings - Energy - Automatically go to pause)/ The cpu fan goes back to silent/off. When you unlock the lockscreen again; the cpu fan stays silent.