The Noctua NF-F12 computer case fan is the quietest case fan of all the ones I tested and I tested a large number.
See http://www.noctua.at/main.php?show=productview&products_id=42&lng=en for the technical details. here is a summary.
The size is 120mm, the largest size you can fit in a standard desktop or tower case.
The speed varies between 300rpm and 1500rpm with my test machine running most of the time at the lower speed because the biggest heat source is the power supply and that has it's own fan. The second biggest heat source is the processor and the processor is not overclocked, leaving the heat output at an easy level for a 120mm fan.
The fan speed is automatically controlled by PWM and PWM is a huge contributor to quiet operation. With an ordinary manual fan control, you have to run the fan at a high speed and noise level to cover the peak heat output. A PWM fan can sit there quietly rotating at a slow speed when you are reading information and only has to speed up when something in the computer starts to work hard.
At 93.4 m3/h, the peak airflow is more than what you need for a processor when not overclocked. I could have twice as many disks installed without generating enough heat to make the fan work hard.
Currently I have 3 computers side by side with the middle one switched off. The one on the left has the NF-F12 PWM fan and it is almost as quiet as the switched off computer. When I am not using the processor or the DVD drive, the only thing indicating the computer is switched on is the front light. If I start a high activity application, the CPU fan is the first noise. The DVD is the worst noise on the rare occasions I use the DVD drive.
The test computer uses an SSD for most files and activity so there is no noise there.
The CPU fan is a replacement fan because the standard Intel fan was too noisy. The power supply is incredibly quiet and only produces noise when the CPU fan is working hard.
From a noise perspective, the DVD drive is the worst, the CPU fan is occasionally noticeable, the power supply and disks only make a little noise and only when the CPU fan is working, and I cannot hear the NF-F12 case fan at all from the front of the computer, not even at the times of peak CPU usage. I am seriously thinking of replacing the fan on the processor with an NF-F12.
The Noctua NF-F12 PWM case fan is currently, in Sydney, only $10 more than a standard 120mm PWM fan and is cheaper than some of the noisier
premium models from other brands. Prices vary from shop to shop by more than than the variation between brands and few shops stock Noctua. You might have to shop online and the dollars you save there might be enough to pay the delivery charge.
The Noctua NF-F12 PWM case fan is so quiet that I am replacing the fans on my other computers with the NF-F12.