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Silent Computer :)

O.K. so you're sick of that constant annoying hummmm from ya computer,
well so was I and now mine is whisper quiet :)

Background:

Basically fan speed, and consequently it's noise is proportional to it's operating voltage. so the simplest way to quieten a noisey fan is to give it a lower voltage.
NB care must be taken that the fan is still doing it's job, COOLING what it's bolted to!!
But trust me, most computers are way over cooled, and thus very noisey.
As i type this my power supply fan is purring quietly and my cpu fan occasionally spins up, and then relaxes for a few mins :) This is because unless you're doing lots of stuff, the 'ol computer isn't generating very much heat.

OK to the methods.....I've used two different techniques to quieten my PC. For the PSU fan I used a string of diodes in series and for the CPU I chose a very elegent and zen temperature controlled fan circuit.

Option 1: diodes

When I designed and built this I was living at home on my parents farm. I was 2hrs drive from the nearest electronics shop, so I used parts off old circuit boards, and diodes were plentiful :)
This is crude, but hey it's very simple to set-up, and I've put a switch on the front panel and can rotate the switch to select how many "diode drops" are in series before the fan. A diode is like a one-way valve or you might prefer to imagine it as a one-way street. Electricity will only flow in one direction, and the 'cost' of this is a small loss in voltage. This voltage drop (typically 0.6v) is lost as heat as electrons overcome the charge seperation in the depletion region of the diode.

When the computer isn't doing much I let it run at about 8 volts, and it's whisper quiet. (this is 7 diode's is series between 12v and the fan)

Cost $$

The diodes I used were salvaged off old circuit boards, and so was the switch, so it cost me and the environment NOTHING :)

I should point out that if you want to modify your PSU fan you will need to disassemble the PSU and you must take GREAT CARE !!
There are many exposed HIGH VOLTAGES within the powersupply!
Each of the large heatsinks will be at around 400volts !!

Yes, this will kill you.

Be VERY careful, and never test it with the case open !!

Option 2:

The method I used for my CPU fan is slightly more complex, but much safer because there are no high voltages.
It involves removing the heatsink and glueing a temperature sensor in place.
It also needs more adjustment to get it working well, but results in a temperature sensing fan control for very little dollars!
If you want a simpler option just use a string of diodes in line with the 12v to the fan. as explained above in option 1.
I'll just give the link here, this will take you to the site, then click on the "Temperature control" link in the menu on the left. Build yourself a fan temperature control
Thanks to Tillmann Steinbrecher for his excellent circuit and full explanations
I thoroughly reccomend it.

Cost $$

The small component count keeps the cost very reasonable, I built mine for about $5.

Here are a few pic's of my results.

OK so this is my PC with wires coming out the front, and insulation tape holding on the all important PSU fan speed selector switch :)

Here is the modified CPU fan setup, bottom right is the tiny veroboard with the FET and variable resistor.

This is a close up of the thermistors. They have been glued onto the CPU heatsink with araldite.(I choose to use two, so one day i can monitor the temperature with one, while the other is operating the fan control)

This hedgehog like device is the heatsink on my motherboard's southbridge chip, it also gets hot, so with less air being blown around i wanted to increase it's cooling ability. I simply bent it's fins apart.
This last bit works well and didn't cost me or the environment anything!!!