If you cannot find an external, physical reason for the click, it is probably the bearings going bad or an electrical issue, I would say probably the bearings. That is your heatsink fan, which is very important in keeping your CPU cool. I would buy a new one and replace it, you will simply have to unfasten it (it probably has small metal clips), taking note of where it is plugged in (probably a header labeled CPU-FAN), you will then clip the new fan on and plug it into the previously occupied header. Obviously, make sure the PC is off and that you ground yourself before touching anything. Also, fans are pretty cheap, but the more you pay the higher air pressure it will provide and lower noise levels as well.