In our noise level tests we switched off all the case fans and replaced the AMD Radeon HD 7950 graphics card with a HD 5550 passive model so were just taking into account the noise generated by the cooler fans themselves. We measured the noise level with a Digital Noise Level meter at a distance of 1m from the front of the case.
In quiet mode, the H100i is perfect for those looking for a quiet cooler as it is barely audible. When you select the higher fan speeds the noise becomes slightly more intrusive but not unpleasant. At maximum fan speed, the noise generated makes you wonder whether or not you have some kind of aircraft taking off under the desk.