I have an oldish PC (Athlon XP 2200, WinXP). Every time I switch it on, I hear a loud "click" first of all, and when it is shut down, again a click is the last sound I hear.

Lately it started to behave erratically: it started to click loudly in the middle of a session. First only once in a while, then repeatedly for several seconds in a row, and finally it froze completely. We were not doing anything particular during these times, usually just browsing the web. This has already happened twice in a few week's time period. We typically use it only in the evenings, so when the freeze happened, I just decided it is time for the bed. When it was started up the next day, everything looked fine.

Any hints on what could be the culprit? Could it be caused by an ageing heat fan, or dust that has accumulated inside the case? We are backing up all the data stored on it, and then will open the case to look inside, but I thought it would be good to get some background info first of all.