Something was going "wheep wheep wheep". It was starting to annoy me. I checked the phone - not off the hook. Then I realised, it was tania, the computer I use for having loads of terminals open to various servers. Tania was telling me "I'm overheating".
So I powered it down, and opened it up. There's four fans, One is in the power supply, that was fine. One was at the back of the case, that was fine. One was in the front of the case, and that was too stiff to turn. And one was blowing cold air onto the CPU, or rather it wasn't, because that was only just about managing to turn slowly. And it was the last one that was probably the reason for the wheeping.
I got out my several boxes of spare fans, and replaced the two faulty ones. Those were my last two 60 by 60 by 15 mm fans, so I went on to Ebay and ordered several more for stock. I powered tania up and put it back into service, and there's no wheeping now.
In a computer, the cpu fan is the component most likely to fail and stop the computer from working. It's a mechanical thing, and they wear out after some years of service. They're very cheap (I just paid £0.99 for these fans) and I expect they're built to the price.