Peak and Floor levels

Noise isn’t a Pass/Fail thing. You should pay attention to the actual numbers. The ACX Noise limit is -60dB. As long as your noise numbers are quieter than, say -65dB, you should be good to go.

https://forum.audacityteam.org/t/audiobook-mastering-version-4/45908/68

If most of your noise settles around -62dB or -63dB, that’s too close to the limit and you risk having some chapters make it and some not.

When you run ACX Check after mastering, what are your noise dB numbers?

Koz