A very important component of most PSUs is the fan that handles cooling. However, there are some passive PSUs that don't utilize any active cooling. Fans keep sensitive components (like electrolytic caps) at appropriate temperatures. Doing this prolongs the PSU's life span, but the type and quality of the fan also plays a key role in its noise output. The circuit that controls the fan is responsible for its speed, and thus acoustic profile, under various conditions. If a manufacturer uses a high-speed fan, chances are that it will increase the overall noise output, especially at higher loads.
Many high-efficiency (Gold-, Platinum- and Titanium-rated) PSUs nowadays feature a semi-passive mode during which the fan isn't utilized at lower loads, and in some cases even at mid-range loads. The PSU is totally inaudible in these load regions because the fan starts to spin when temperature inside exceed a specified threshold. This temperature is hard to reach under light-load conditions due to the increased efficiency and low energy dissipation.
To implement an effective semi-passive mode, especially in high-capacity units, PSU manufacturers use large heat sinks. This ensures that the active components are able to effectively dissipate heat, especially when the fan-activation threshold is set at high levels.
Since in many cases the semi-passive mode operation allows the build-up of high internal temperatures, we aren't very fond of it, at least in PSUs with fans that can start at low voltage levels. In our humble opinion, a fan rotating at low RPMs will be much more effective at keeping the temperatures at normal levels. On top of that, if the manufacturer chooses the right fan model, then it can be totally silent at such low speeds.
Unfortunately, with some units (for example, high-capacity PSUs that are equipped with powerful, high-speed fans), even in the best-case scenario output noise is increased. That's because powerful fans have a high start-up voltage, which means that at their lowest supported voltage, they spin quickly, generating enough noise to be annoying for some users. In such cases, semi-passive operation can make a true difference at light loads.