Fan in PC revving up and down?

[benwinwood6]

I have had a sort of revving noise in my PC for quite a while, I assumed it was the Cpu or Gpu and since I have replaced both the Cpu fan and tried two different motherboards.

I remember having the issue on and older PSU also, and have since upgraded to an Evga G2 Gold.

I have turned unplugged all external fans so only the Psu and Cpu are running but I can't hear which is the source, even with my head close.

It revvs up and down every 5 or so seconds, not heavily but enough for it to be really annoying.

What is causing this and how can I troubleshoot it?

 

[SkyNetRising]

Check your motherboard bios. It should have some form of cpu fan adjustment.
Either set minimum cpu temperature to a different target or set minimum fan speed to a different target (higher).