The fan on my Thecus N2100 is getting a bit noisy. I am sure it is starting to fail. Trying to get hold of a new one is a nightmare. However, I have eventually found a potential replacement at Maplin.co.uk. I will let you know if it all works.
UPDATE: The above fan from maplin.co.uk works a treat. Fits perfectly (had to tie up the cable to keep it neat and tidy as it is a bit long). It’s brushless, very quiet, has the right connector and works with the two speed settings in the thecus admin interface, even reporting the RPMs. It was a £8 gamble and it paid off 🙂