AlanRobertson has an interest in quiet(er) PCs. As you can tell, he has a lot of them, and having the ones near humans be a little quieter would be a good thing. He also is thinking about making a personal video recorder, which would probably wind up in the living room, and noise would be quite annoying.
The basics are:
There are basically three things that make a hard drive audible:
If the drive is spinning, the motor typically makes a little noise. In some circumstances (like a TiVo, you can't spin it down, and it needs to rotate to be read or written. And, even if you can spin it up and down, it's generally thought to be hard on the drive.
As the drive heads move around, the actuator mechanism makes some noise. Most applications that use the hard drive will seek occasionally - and some applications will do this a lot. It may be a clicking, or clunking or other intermittent and not-quite-repetitive noise. Seeking the drive may result in vibration of the case, which can cause noise of its own.
This can be one of the most annoying features of a disk drive. It is usually described as a whine. Most commonly it occurs at higher rotational speeds (10K RPM and above), but some brands of drives whine even though they run at slower speeds.
Generally, slower fans are quieter than faster fans. Higher speed fans typically cause air turbulence which greatly increases the noise made by the fan. No fan at all (passive cooling) is best when you can do this. Improving heat sinks sometimes allows for passive cooling, and sometimes it simply allows you to run a slower (quieter) fan. Switching to a larger fan typically allows for a slower fan speed as well. Slowing down CPU speeds (underclocking) typically generates less heat as well. For most CPUs, it's the CPU speed (not the bus speed) which determines heat generated. Most power supply noise is fan noise. You may be able to replace the power supply fan with a slower one if you aren't using the power supply to its full potential and the environment you're using it in a fairly cool room. Some power supplies do not require fans - but they're typically expensive. Quite power supplies with fans when done right, are typically power supplies with good heat sinks which run fans at slower temperatures.
Slowing down fans typically increases the temperature of components. This is why in such PCs doing things like switching to round cables instead of flat cables can help improve ventilation, which may allow you to run a slower fan speed.
Quiet cases are cases that have some or all of the following features:
Links from Acousti Products
SilenX (quiet fans, power supplies),
SPCR - Silent PC Review
ATAcom.com - Quiet Solution