This is usually normal. Terrestream schedules a SEN66 fan self-clean once a month, on the first Tuesday of the month, around 10 AM local time.
It should be brief. The cleaning cycle lasts roughly 13 seconds and may show a cleaning animation on the light ring.
There is a cooldown. The firmware waits at least 24 hours between cleanings. You can also start a cleaning from the app's device settings over Bluetooth.
Contact support if it is constant. A short sound once a month is expected. A fan sound that repeats often, never stops, or appears with other device problems is worth reporting.
