A Mac mini running Plex (or Jellyfin, or a small home-lab stack) spends most of its life doing nothing much — serving a library index, waiting. Then someone starts a 4K stream that needs transcoding, and for a few minutes it's doing real work. macOS's automatic fan curve treats both states about the same, because it's reacting to temperature, not to what's actually running. If the mini sits somewhere you can hear it, that gap is what you notice.
Why "just make it quieter" isn't the whole answer
You can't just lower the fan curve globally — a Mac mini transcoding several 4K streams at once genuinely needs the airflow, and macOS's own thermal failsafe will always override anything quieter than that once temperature actually climbs. What you want isn't a quieter Mac mini, it's one that's quiet by default and reacts on its own the moment there's real load — without you having to notice, log in, and change anything by hand.
Trigger on load, not on the clock
FanFan Control's automatic rules react to two things: sensor temperature, and whether a specific app or process is running. For a headless server box, the process rule matters more than it sounds — it doesn't require the app to be in the foreground, just running, which is exactly the case for Plex Media Server or its transcoder doing work in the background while nothing is on screen. Point a rule at the transcoder process (or at a CPU temperature threshold, which reacts regardless of which process caused it) and the fans ramp up only when there's something to cool, then settle back down once the stream ends.
What FanFan Control doesn't have — deliberately, not as a missing feature you're waiting on — is a scheduled or time-of-day rule. "Quiet after 11pm" isn't something it can do, because it isn't watching the clock at all. If what you actually want is silence overnight, the rule that gets you there is a temperature or load-based one: a mini idling overnight has nothing transcoding, so it stays at its quiet baseline on its own. Write the rule in terms of what the machine is doing, not what time it is, and the two end up the same in practice.
Whatever rule you set, Apple's own safety margin stays in place underneath it: above 95 °C, every fan reverts to fully automatic within three seconds, and that can't be disabled.
Set it once, then forget it's there until it needs to react.
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Any questions?
Write to soporte@fanfancontrol.com and I'll answer myself.