I've just finished building a new computer with one of those new-fangled Haswell CPUs; in particular, the Intel Core i5-4570.
I plug in the power, turn it on, and watch as everything starts spinning up right away... except the stock CPU cooler fan. Instead, in the first 5 seconds or so the CPU cooler fan simply twitches a couple of times, after which it starts up with a loud whir for 2 or 3 seconds, before spinning down to normal speed and quietness.
Being a first-time PC builder, naturally I got worried that something had gone horribly wrong with my CPU cooler, especially seeing how the fan blades were twitching in the first few seconds. But apparently, this is normal, and something that many motherboards (including my ASRock H87M Pro4) do by default as part of their fan control procedures, so I'll assume that it's safe. I'm not overclocking anyway.
But what's the purpose of this behavior? Why doesn't the CPU cooler just spin up normally like all the other fans (case, GPU and PSU)? Also, if I can change it so that the CPU cooler fan does spin up normally, should I do that or is it better to leave it as it is with this seemingly unusual behavior?