Sony FX 6 Quick Start Manual page 38

Audio: Fan Noise
The sensor is large and requires cooling (or heating).
Menu/Technical/Fan Control:
AUTO mode is the default for the fan. In this mode the fan may run while
you are recording. If the microphone is on the camera, or near the fan, it will
record this noise.
The two other options are: MINIMUM and OFF IN RECORD.
I recommend OFF IN RECORD if you are shooting in a cinema style with short
takes. The camera will have time to cool between takes.
MINIMUM will work better for long takes/documentary/interviews. The
noise will be consistent but low. Consistent low fan noise is much easier to
remove from an audio track than a noise that varies in volume.