Here is a little background on how we hear and how we measure how loud something is.
Sound is merely the vibration of a medium, most commonly air, causing waves of pressure at different frequencies. These waves are interpreted by our brains through receiving the waves on the tympanic membrane (eardrum), which channels the vibrations through tiny bones to the vestibulocochlear nerve. You can see how air-cooling a PC is never going to be truly silent. As long as you are moving air across an object forcefully, you will have a disruption of clean airflow which is, by definition, noise. Quite a catch twenty-two; more cooling requires more air movement across a surface.
The common measure of sound pressure is the decibel, or dB. This non-linear scale gives precision to lower levels while allowing for a wide range of measurement in a fairly compact scale from 0 dB for pure silence to 180 dB for removal-of-your-ear-drum levels. The scale the dB uses is a base10 logarithmic scale, much like the Richter scale, meaning that 20dB is more than twice as loud as 10dB, and is contingent on the distance from a given sound source. To be even more confusing, the scale does not behave the same for every frequency. The scale can be weighted in an attempt to emulate how our ears attenuate the highest and lowest frequencies which causes most sound pressure data to be given an extra character. The sound pressure levels (SPL) is commonly dBA as the A scale is most prevalent. The more recently accepted C scale creates a more linear chart when comparing relative dB levels betweens frequencies, yielding a dBC designation.
Here is a list of common sounds and their associated approximate SPLs.
Air Raid Siren
I have tried to quantify and create samples of the noise output of each step of the upgrade process. First, I used a dB meter at a distance of one foot. Please note, the number on the gauge is merely a relative measure only used to convey the progression and validate what I can hear. In order to provide a more tangible benchmark, I used a Sony Hi-8 video camera and captured video and, more importantly, audio. The line-in levels, the distance from the system and the environmental conditions were unchanged between recordings. For example, I ensured air conditioning was not running during measurements. This should help to convey the relative sound level differences with the case wide open.