Random fan spin up Inspirion 3737

Dear Community Support,

we've bought an Inspiron 3737 a couple of months ago and the cooling fan spins up seemingly randomly in every x minutes. I say randomly because I was unable to find any link between the spin-ups and the sensor readings. Even when the laptop is on the desk, doing nothing (lid closed but running) we can hear the spin-ups.

What is the reason I think is that there are only 3 states of the fan controller to set the RPM: 0, 2500 or 4900

Is this normal? By design? You can see on the screenshot, Prime95 is running in the background and the CPU temps are OK and the fan is on 2500 RPM which is fine.

Any advice?

Thanks in advance,

Marton