...Well, as well as case fans, heatsink fans and the PSU fan (if there is one) when the PC is on, your Hard Drive(s) are spinning which also create a sound.
If you're certain that it's one of your fans, and you know which one it is, you could try lubricating the fan by removing the sticker on the back, and adding a drop of 3-in-one or other, or you could buy a new fan and replace the noisy one.
If you still are not certain which fan it is, I suggest you remove the case and listen closely to identify where the sound is coming from. Briefly stop the fan for a moment to tell if it is that fan which is making a clicking sound (you may be able to do this outside of the case).