OK, some things I have learned about electronics (from a starting position of zilch a few days ago);
1. The fan in the Kessil is a fairly standard 40x40x20mm affair. Physically very easy to find a replacement.
2. However, it runs at 24v instead of 12v like most computer fans.
3. You can run the standard fan at half speed or a 12v fan if you wire in a LM7812 voltage regulator. This is very easy to do - ideally involves a spot of soldering or you can bodge it like me with electrical tape.
4. The half speed stock fan still pushes more air than the 12v northbridge fan I tried.
5. Doing the above will drastically reduce fan noise, and runs the unit cool enough not to cause any issue (short term at least). However, the unit does run considerably hotter so you're probably shortening the life span of the LEDs from the rated 13 years to...... who knows.
6. Running the unit without a fan at all causes it to switch off after about 10 minutes when it gets too hot. Good safety feature.
7. Accidentally crossing the red and black wires going to the fan whilst its switched on causes a burning smell and the death of a fuse on the circuit board, which renders the entire fan circuit kaput.
8. After a lot of swearing you can run a second power cable for the fan directly to that old mobile phone charger that's been sat in your drawer for about 8 years, because it just so happens to be 12v DC.
So in summary; if the terrible buzz from the Kessil fan is setting your teeth on edge, you can easily fix it by wiring in a 12v regulator as detailed here - AVForums.com - View Single Post - Behringer amps, x-over and parametric EQ units and hence running the stock fan half speed, or replacing with another fan of your choice. The downside is that the unit will be hotter, probably reducing its lifespan.
Can't help but feel Kessil should have made the case large enough to fit a 60mm fan and larger heatsink; would have made cooling much easier and quiter with only a minor increase in bulk.