I'm currently piecing together a new second hand build. It's my first system build since 09/10 when I built my current X58 system. Back then, I never used PWM fans, but for this new build, I'm using the NH-D15 which comes with PWM fans. While testing, I plugged the fan into the CPU Fan header, and when I entered the BIOS, noticed that it was spinning very low RPM, around 400 or so. The CPU temp said it was around 21C. On some occasions, when I booted up, the POST would halt, and I would get a CPU fan error, but I'm sure that's due to the fan running at low RPM. I did find an option in the BIOS to disable low RPM monitoring or something like that. So my question. How are PWM fans designed to work, and is it normal for them to run at this low RPM? Thanks.