Required parts and tools:
Level of difficulty: easy
The fan used in the GuruPlug makes a lot of noise. I was wondering if running the fan at a lower speed could lower the noise and still ensure the required cooling. The first step was to improve the air flow and the cooling effect since the fan was placed in a very bad position. The second step, which is described here, is to make the fan run slower to decreases it's noise. Using some temperature measurements I made sure that the fan does not run too slow and that the important parts are cooled enough.
Slowing down the fan is very easy by limiting the fan current. Using the given data of the fan, the resistance of the fan motor has been calculated as 41.67Ω. My first choice was to use a resistor with similar value, connected in series, which would halve the current. I then decided to use a 39Ω resistor to be save (read the warning).
Even better: Instead of using a resistor, a red LED is a better idea. The red LED gives a drop of about 1.8-2V which is a good value for the used fan. I tried to use different LED colors with higher voltage values, but it seems like a drop higher than 2V is too much and the fan does not run any more.
The noise seemed to be acceptable, even though it was still too loud to let the GuruPlug run over night when sleeping in the same room. Temerature measurements showed that with improved air flow and the slower fan speed, the cooling was ok (60°C / 140°F at 100% CPU-load).
Adding the resistor/LED
The fan can be easily unplugged after opening the housing. Cut the red wire apart (about in the middle), remove a small part of the wire insulation and solder the resistor/LED on at one side (take care of the LED polarity!). Now, do not forget to slip the heat shrink tube over, and then solder the other side of the resistor/LED on. To shrink the heat shrink tube, a easy method is by holding the flame of a fire lighter close to the tube. Take care that no other parts of the GuruPlug are close to the flame, and shrink parts of the tube step by step.
When using the 39Ω resistor, my fan stopped running after a while. Just by touching the fan and thus giving it a little push it started running again (the GuruPlug-housing was quite hot already). I then replaced the resistor with 22Ω which resulted in a higher fan speed (and it was much louder too), but the fan did not stop any more. A higher resistor value of 27Ω might also work. A comment of Confusticated gave me the idea to use a red LED, which does not limit the current but drops the voltage by about 2V.
Since my GuruPlug is now running outside of my room, the noise is not that important any more and using half the speed is silent enough. A slightly higher resistor value could be used to decrease the noise even more, but care has to be taken to prevent overheating! My temperature measurements might help when choosing a different fan speed.
Another possibility would be to use a different fan. The fan used here is a ball bearing fan. A sleeve, magnetic (maglev), or fluid bearing fan would probably be more silent.