Strange fan speed

I noticed that my 2 main fans(cpu and case) seem to speed up at random times. I can be running toast for 30 min and nothing, the cpu temp is always under 40C and then when I am just browsing the internet and m y cpu is at 8% the fan speeds up. I cannot control the fan with speedfan, it is automatic and they usually spin at 42% they can speed up to 5k RPM is there a way I can control them myself, preferrably without buying a fan controller, the only temps that ever get hot are GPU and AUX according to everest.