hmm, could be also the fan is stuck, and then it gets free, I would open up your case, if possible, and run it open, then when you hear the noise put your finger on the fan, in the middle, not on the bladed area, apply a little bit of pressure to each one that may be suspect, and listen for a change in the sound. same process should be done to any other suspect area, not fan related. No you wont shock your self, and no you wont burn your self, CPU heat sinks only get up to about 140F everything else wont be above 120F, and if it looks like a exposed wire just don't touch it. and as always ground your self first, hold the case while doing this.
as a disclaimer, this should only be done by professionals, where safety glasses

and if needed consume alcohol first to clam your nerves.
The views expressed by this poster are not endorsed by this poster, there I think I covered everything.