Computer just did something strange.

[MuGen]

I've experienced many a clicks in my time, and have since opened up the harddrives. It appears the click sound comes from a little needle that comes loose everytime your harddrive is over reving, or going over it's rotations per minute.

It's nothing serious, just turn off the cpu entirely and turn it back on. Best advice is to get a new harddrive, slave your old one and copy all your data.