(Up to user-created-add-ons)
The Vanilla and Duo Bukobot models come with a 350W fan-cooled power supply. Unfortunately the stock fan in the power supply is a little on the loud side and the fan speed control seems to be non-existent.
The following instructions are to replace the fan with a quieter model. The power supply still makes some level of noise but it is moderate improvement compared to before - with much less of the irritating high pitched whine. (Unfortunately at this size of fan a fair proportion of the noise is actually just due to air movement but better blade and motor design can definitely help).
The stock power supply fan is a 60mm x 15mm 12V DC brushless fan with a simple (but non-standard) 2 pin connector.
I chose the following replacement fan: http://www.ebay.com.au/itm/Superred-60-mm-Quiet-Cooling-Fan-12-V-DC-18-CFM-19-dB-CHB6012-super-red-/271091556557 for $7.80 US (including US delivery).
It is a Superred CHB6012AS 12DV 60mm x 15mm ball bearing fan which produces 18CFM at 19dB.
As you can see in the picture below (the original fan is on the left), the Superred fan has more blades and the blades are longer which means that you can have a lower RPM whilst still maintaining airflow.
You can also see the old fan cable is longer (15cm), has a thermal insulating sheaf to protect the wires from hot components inside the case and has a smaller 2 pin connector. That's okay - we are going to do a bit of cut and shunt on the fans.
[Requires a soldering iron.]
1) Unplug the power supply.
2) Open power supply case by removing the 6 screws around the top edge.
3) Disconnect the 2 pin fan connector so you can fully remove the lid.
4) Unscrew the fan from the power supply cover.
5) Bunch up the thermal sheaf towards the connector end. Hold with a clamp or bulldog clip.
6) Cut the wires a 2-3cm from the fan. I cut them at slightly stagger lengths so that the solder joins can't touch.
7) Similarly cut the wires on the new fan.
8) Strip back a short bit of insulation and solder the old connector to the new fan.
9) Put some Kapton tape around the solder joint (probably overkill in this case)
10) Remove the clamp and reinstall the fan into the power supply lid. Reconnect.
11) Close up the power supply, carefully reconnect mains and turn her on!
Do not connect the power supply to the mains without taking adequate precautions in ensuring that electrocution cannot occur through accidental contact with the mains terminals, cables or internal power supply components.
The original fan was measured at 79dB (using a Radio Shack meter) at 2 inches. The meter was set to the side such that airflow was not blowing directly over the microphone Using the above instructions, the sound dropped to 65dB using the same procedure. In an effort to further reduce sound, the fan was then reversed so that it was pushing rather than pulling air. This dropped the measurement down to 60dB (note that the change in airflow could impact the measurement, but the power supply was qualitatively quieter. Lastly, the fan guard was cut out, exposing the fan directly. The final result was 55dB at 2 inches. The supply is still louder than I'd like, but it is a huge improvement. The next step would probably be to fabricate a new top-plate that can hold an 80mm fan to allow slower speeds and quieter operation.
Another user has connected a larger fan onto the top of the power supply to replace the stock internal fan. This has two advantages:
The obvious disadvantage is that the supply no longer fits underneath the Y-axis which may or may not be a consideration for you.