What is the reason for the periodic increase in cooler speed?

Question

Iron

Desktop - "mini" computer (mATX).
Proc i3, 8 GB memory, sdd. Everything. The only cooler on the processor.
The screw is new. I recently changed the paste (the temperature of the cores dropped from 45 to 30).
Now
SpeedFan
sdd is 48 degrees.
Cores no more than 30 degrees.
Despite this, more often in the afternoon, the cooler starts to behave strangely.
Accelerates to high RPM for 2 seconds and drops RPM sharply to normal values.
Fast acceleration - 2 seconds and a quick return to normal values.
And so from 2 to 15 minutes.
(Something like how the car accelerates. This is for comparison.)
As I wrote above - the screw is new (the old sdd worked for 3 years and died), there is no dust inside. Containers are not swollen.
The computer starts up quickly. The work is stable. Cooler cleaned. Changed the thermal paste. There is an effect - the temperature dropped to 25-30 degrees. The air flow from the cooler is slightly warm.
And here the cooler does such tricks. It is a quick acceleration for 2 seconds (it spins quickly and sounds loud) and a quick return to normal values.
Usually the cooler spins fast (and therefore sounds loud) when the integrated graphics are loaded.
But there he does it all the time while the application is running. And the core temperature is rising.
Immediately, periodically, "in jerks and just like that" - without any connection.
What is the reason for the periodic increase in cooler speed?
Maybe there are other factors? Voltage for example?
No UPS - enterprise, it can jump (sometimes they turn off the electricity so anything can happen and jumps)

Answer 1

processor load. if you don't explicitly call anything, it means background processes. task manager to help, you can also use the resource monitor.

Answer 2

also look at the settings in the BIOS - often the mode of operation of coolers is described there

Answer 3

An instant rise in temperature by several degrees, and then its fall - the processor does not have stable 36.6 - the MP reacts accordingly, increasing the fan speed, and immediately lowers it when the temperature drops. That's the effect of "gassing". The solution is to manually adjust the dependence of the fan speed on the temperature, to make the reaction less sensitive. Track within what range the temperature dances in normal mode, using, for example, AIDA64, the "Sensors" section, and set the limit for increasing the fan speed in the BIOS. That is, for example, t of the processor jumps in the normal mode from 40 to 50 degrees. Then we set, for example, 20-30% of the fan speed to 60 degrees, and above - a sharper increase in speed. Thus, in office mode, the fan will not "gas",

Answer 4

Most likely the cooler control software does not work or something else, in fact, the speed depends on the voltage and intensity of work, look in the BIOS settings and special programs for setting up Coolers