Reply 20 of 36, by songoffall
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I know "completely silent" would mean using passive cooling and solid state components, either not viable or not worth it in most cases. But then, when we talk about silent we mean silent compared to the ambient noise level or speakers, silent enough that it doesn't distract you.
At this point, I've observed several kinds of fan noise.
1. Bearing noise, especially if the lubrication has dried/the fan is dirty/older fans, especially with ball bearings, and about any fan at max speed makes bearing noise. Using better fans usually helps with this, but I also want to experiment with thermistors to control my fan speed based on temperature - old PSUs and motherboards don't have the modern ethos of fan speed control.
2. Air turbulence, especially when a high CFM fan pushes against an obstruction, bad cable management, fans against components without enough clearance. This can be fixed by removing airflow obstructions like the poorly perforated exhaust ports on older PCs and replacing them with wire-based grilles. Also, adding clearance between the fan and the exit helps a lot too.
3. The case itself vibrating. Especially with cheaper and flimsier cases with bendy side panels. And with aluminum cases. The more mass your case has, the more energy it requires to move, so it vibrates less. Rubber screws and anti-vibration pads are a lot of help too.
Of course old PCs make other kinds of noise. The HDD can be noisy, especially older models. The optical drives, the floppy drive. The coil whine, especially from the PSU. Coil whine, as already noted, can be dealt with with electronic grade silicone - power capacitors, chokes, VRMs, using silicone to absorb the vibrations has proven to be quite effective, but you also don't want the material to turn into a conductor.
Compaq Deskpro 2000/P2 300MHz/384Mb SDRAM/ESS ES1868F/Aureal Vortex 2
Asus A7N8X-VM400/AMD Athlon XP 2ooo+/512Mb DDR DRAM/GeForce 4 MX440/Creative Audigy 2
Asus P5Q Pro/Core2 Quad Q9400/2Gb DDR2/GeForce 8800GT/Creative X-Fi