is it such a big deal to use the noise reduction tool to remove that fan sound
Not if you remove the fan, no. From memory, your mastered and DeEssed clip comes in at around -62dB and the spec is -60dB. So, yes, it passes… but if you change anything or do anything wrong, it’s going to fall outside of the specifications. ACX uses an automated testing process similar to ACX Check. So it’s going to go ripping down your whole book looking for that one chapter that didn’t make it.
There’s a process problem, too. New Users check a box and chapters that just make it over the line check another one. They’re gunning for you.
Home recordists never pass noise. We live in insanely noisy environments and nobody has any good idea what quiet is. The specification of -60dB means your background noise has to be 1000 times quieter than your voice. Nobody’s living room can do that and even my super quiet bedroom fails when the Metro Bus goes by. They didn’t just pull that out of nowhere. That’s the broadcast specification.
If that noise is your computer, you have one advantage the USB Microphones don’t. You can make the cable between the NT1-A and the interface can be as long as you want. 50 feet? No problem. You can link them and extend them, too. It’s the USB cables that should not be longer than about 6 feet (2M).
You should also note that ACX will only flag the first problem they find. The first rejection. You could have more problems. It’s not their job to troubleshoot your studio. There’s another fuzzy rule, too. If they start complaining about your voice, then chances are good they didn’t find any mechanical problems—and they will usually say so. For example: “The recording quality is good, but we didn’t like your lip smacks.”
Noise Reduction of the Beast (6, 6, 6) gently reduces the background shshshshsh sound and can’t be detected in normal operation. It doesn’t affect your voice. Stiffer reduction than that, say 9, 6, 6 makes us ask what you can do to improve your voice without the reduction. By the time you get to 12, 6, 6, you can hear the reduction working in your voice and the correction is to be avoided. You need to fix your studio.