I needed to change the mounting I have described in this article. More on that here:
The Silence of the fans
I understood early that I needed some active ventilation if I wanted to have my HTPC inside a cabinet. Of course this ventilation needs to be silent.
You can read about the cabinet here:
Building a cabinet for the living room to house a HTPC
This is the solution:
A 120 mm fan running at 220 V, speed adjusted with a special device for… well… adjusting fan speed…
I tried mounting the fan directly on the cabinet. No matter how slow I ran the fan it would make a very clear noise. So, I had some kind of idea of how I could mount the fan using rubberbands. I have silenced all my hard drives using floating systems like this.
Last weekend I had my parents visiting, and my father was helping me finishing the cabinets. I told him about my idea for the fan. Half an hour later he had made the following arrangement:
The fan is completely silent, it pushes huge amounts of air, and I have never had a lower CPU temperature than what I am experiencing with my computer inside this cabinet.
Here is a video clip showing the functionallity:
Link to video with silent fan – Windows Media
Edit: Here are some more pictures of the attachment of the fan (click them to enlarge):