Metal fatigue takes years to occur and as these small fans are generally attached via plastic clips - the clips may have broken or not been properly attached in the first place. Many new fans need you to push the clips through the board first and then press a (usually black) plastic stick down through the middle to open the clip out to stop it coming loose. Check yours and make sure it there's a piece of plastic down the middle of the clip.
Your local PC fixer should be able to diagnose immediately.
There was a small magnetic sort of washer stuck to the edge of the fan, so I put that over the fan shaft, and gently pressed it home. It's behind the power supply and very difficult to get to or even see properly, but it seems quite securely in place now and all noises have stopped. I'll tick as resolved, as I'll know what to look out for if I get the noises again.
Thanks for your replies. :o)
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