With several Zotac Zboxes, there is no way to set the speed of the system fan in BIOS (only CPU fan can be configured in BIOS).
Workaround for Windows OS:
- Download, install and start Speedfan. (Note: Some shown temperature values may be wrong, but that does not matter in this case)
- Adjust „Sys“ value on the start screen to get a value like e.g. 65 % that fits to your system (don’t set it too low).
- Set checkbox „Automatic fan speed“.
- Click „Configure“ -> Speeds -> Sys.
- Set a lower minimum and maximum value.
- Check „Automatically variated“.
- Create a new scheduled task in Windows which starts (install dir)speedfan.exe at system boot, with full privileges, independent of user login. You can also set a delay of 30 seconds.
Now your Zbox should be almost silent when the Windows boot process is finished.