Since I researched and answered this question in a support ticket, it might be useful information generally. If you

cat /sys/class/thermal/thermal_zone0/policy

, you will find that fan control is set to "stepwise". The method of PWM fan speed control and speed steps are defined in the the device tree. The process of ramping up and down steps are controlled by a thermal sensor on the RK3588 and pre-defined fan speed trip points. If you

cat /sys/devices/platform/pwm-fan/hwmon/hwmon7/pwm1

, you will see that the default or lowest fan step is 100. I experimented with this FIFO a bit. Basically, just through echoing values to pwm1. The possible range of settings is 0 to 255. At 0, the fan is off. The lowest setting that my test module fan will spin at is 12. Setting pwm1 to 255 caused the fan to spin all the way up and then down to the default step. To me, that means the maximum step value is 254. If you play with these settings a bit, you quickly find that a setting of 254 is, relatively speaking, quite loud.

I set my Noctua fan to 100 with echo 100 >/sys/devices/platform/pwm-fan/hwmon/hwmon7/pwm1 , seems enough and no noticable sound. Put it in a init.d script to have it permanent. Thanks for pointing out the path to the device.

That's the default that is set at boot time.