Hi,
I noticed some strange behavior on a CRS328-24P-4S+RM running RouterOS 7.23.3. The same issue also occurs on 7.23.2.
If I shut down the switch using System/Shutdown, the device shuts down normally. However, after approximately 15–20 seconds, the fans suddenly ramp up to maximum speed and remain running at full speed.
Has anyone else noticed this on the CRS328-24P-4S+RM?
Have you ever tried the same with an older ROS version?
I mean, the shutdown makes the ROS version irrelevant as it is not running any more, so there is no control loop that adjusts fan speed to the temperature, but the power supply is not cut off. It is quite common that the fans switch to full speed if they stop receiving the control signal, as a precaution against the equipment overheating if the only thing broken is the control of the fans itself.
You can observe the same behavior with most servers as you power them on - they usually "prepare for takeoff" first until the BIOS/UEFI wakes up enough and starts telling the fans how much airflow is actually needed.
I like your description 
, I used to have a couple CISCO devices with an external PANDUIT PoE PSU that were very similar to a F14A with full afterburners taking off from a carrier.
My HP servers with 10+ small high speed fans are notorious for this . . . . Pretty hard to miss a restart!
I mentioned the RouterOS version because this didn’t happen before (7.22.X) — it only started after upgrading to 7.23.2 and then 7.23.3.
And anyway, it seems strange that the fans suddenly go to full speed after a normal shutdown.
If this behavior did not exist before the upgrade, I would speculate that 7.23.x doesn't shut down the power supply as the last step of the shutdown process whereas the previous versions did. So the way to go is to file a support ticket - forum users can only respond your initial question (am I the only one to experience this) but cannot resolve the issue.
Do they scream as well immediately on powerup?
If no, likely not the controller, but rather MikroTik added "setFanSpeed(BallsOut)=True" to the shutdown code 
(and it holds the last setting).
Thanks everyone, that makes sense. I'll open a support ticket with MikroTik.
On power-up or reboot the fans normally go to full speed for about 2-3 seconds, then smoothly drop back to minimum speed.
OK, so controller default is full speed then . . .