Nothing to worry about.
The fan speeds are an actively trying to cool the MoBo devices.
That is, as the selected primary and secondary control components (CPU, PCH, Mem, VR, etc.) change temperture from their designated setpoints due to there processes and utilization the fan will change speed to support cooling the device.
Also, the variation you are discussing is only 4% fluctuation.
Thanks for the answer. I was worried about the noise from the fan and I thought that this is due to changes in speed. But now put another fan and everything is OK. And the speed of change in the same way.