The end result is a resounding success, I have to try very hard, and often fail, to be able to hear it and I'm in a very quiet room. Plus it cost me very little to build, never a bad thing, and I enjoyed the process.
The key features are:
Low power cpu with a large passive heat sink. Pretty basic really, heat = noise.
2.5" hard drive in custom enclosure (the part I'm most proud of). I made a simple box out of 3mm aluminum then filled it with a mixture of brass swarf (the shavings that come off when machining) and flexible silicone sealant. I placed the drive in a plastic bag, put it in the middle and let it set. I was aiming to make something that would conduct heat but insulate sound and vibration, and I succeeded, particularly with the heat, drive is currently at 32 degrees and was at 28 when the front fan was on. Noise is reduced significantly though a slight whooshing noise can still be heard and vibration is all gone.
There are several large improvements I can think of with the making of this but I think fundamentally it is a very good idea. I wrapped it in lead a few days ago to try and get rid of that last bit of sound and it has helped a little.
Indirect air path in non resonant, sound absorbing case I started with a very simple wooden box as it is easy to work with and will absorb vibrations. the indirect sound path was one of the last features to be put on and made a surprisingly large difference. Definitely something to work on with the next build.
The hardware is
Athlon 3500+ Venice core
Asus Socket 939 motherboard
2Gb OCZ ram
Zotac 9400GT
WD Scorpio Blue 250Gb
ebuyer value psu with forgotten model scythe fan
scythe 1200 fan @ 3-4V
Nexus fan at 7V when used
Scythe Ninja
Not the fastest machine but I had to get rid of my opteron 175 when I learnt what TDP meant.
Temps are CPU - 26'C Idle, 52'C Load, HDD - 32'C GPU - 43'C Idle
![Image](http://img188.imageshack.us/img188/9837/image050n.jpg)
Front intake withindirect airflow path and foam
![Image](http://img10.imageshack.us/img10/8004/image052tfu.jpg)
Hard drive covered in lead
![Image](http://img301.imageshack.us/img301/2170/image053u.jpg)
Rear view
![Image](http://img261.imageshack.us/img261/6060/image055d.jpg)
Front intake fan, soft mounted to an old fan chassis via some rubber foam to give the air a longer path before it hits the front plate
![Image](http://img524.imageshack.us/img524/3595/image056u.jpg)
Side view
![Image](http://img411.imageshack.us/img411/7640/image058e.jpg)
Hard drive enclosure being built
![Image](http://img44.imageshack.us/img44/42/image036g.jpg)
Hard drive enclosure before being wrapped in lead
![Image](http://img301.imageshack.us/img301/513/image045l.jpg)