Certain power supply units have a zero RPM mode. This feature allows for the PSU fan to be completely off when it is not needed and will automatically turn the fan on when your PC reaches a certain temperature.
Once the fan is turned on, it starts at a low RPM that gradually increases with the temperature. The default behavior is listed in the user manual of each PSU with this feature.
If you have an i-series PSU (for example, the AX1600i), you can enable zero RPM mode in iCUE under the Cooling tab.
If your PSU has zero RPM mode but is not iCUE-enabled, you will not be able to turn on or control this feature.
The following PSUs have zero RPM mode: