The fan speed depends on the percentage of CPU usage. I can actually hear my fan speed increase when I move the mouse a short distance and then it slows down when the mouse stops. I can hear this even when I have ear plugs stuffed in my ears. I hate it, but I get used to it.

My laptop lets me set the fan speed, but setting it too low makes the computer run unbearably slow. So I set it in the middle, trading some speed for less noise.