If your system fans are running up to high speed when you OC your system and cause it to generate more heat, that is because the inside of your case is getting hot and your system NEEDS more cooling! If you force your fans to run at a fixed low speed for quiet operation, you will also force all your components to run at too high a temperature and cause more rapid aging even if you don't go so hot the system throttles itself for self-protection. The SYS_FAN or CHA_FAN headers use a different temperature sensor on the mobo to guide them. The CPU_FAN header concentrates on the temperature measured by a sensor inside the CPU chip. The automatic fan speed control systems in any mobo are based on keeping the TEMPERATURE of certain components under proper control. You should realize also an important fact. Then you can use its tools to adjust the fan headers if you wish. So instead if you install and use the AI Suite tool, it is a Windows app you can run on your screen while doing other work and thus 'see' their operations in a 'real world' workload.

The basic issue here is that you CAN configure fans headers in BIOS Setup and observe their speeds, but you can NOT do that while doing real work. But you should recognize that AI Suite is ASUS' own utility for fan (and other) optimization. There are lots of models in the ASUS Z390 series, so we're not sure exactly which one you have.

Asus motherboard control fans