Hollywood

https://github.com/Pyrestone/jetson-fan-ctl  works good.
 
I wrote a little script which monitors the pwm fan speed:  show_fan_pwm.sh
 
#!/bin/bash
# script show_fan_pwm.sh
main () {
  while [ 1 ]; do
  echo -n "fan pwn speed is : "
    cat /sys/devices/pwm-fan/target_pwm
  sleep 1
  done
}
main $@
exit 0
 

The difference between armbian and nvidia ubuntu 18.04 is:
nvidia ubuntu shows in /sys/class/devfreq/ 2 links:
lrwxrwxrwx  1 root root 0 Jan 28  2018 54340000.vic -> ../../devices/50000000.host1x/54340000.vic/devfreq/54340000.vic
lrwxrwxrwx  1 root root 0 Jun  5 18:40 57000000.gpu -> ../../devices/57000000.gpu/devfreq/57000000.gpu

armbian 20.05.5_Jetson-nano_buster_legacy_4.9.140_20200528 shows in /sys/class/devfreq/ only 1 link:
lrwxrwxrwx  1 root root 0 Jun  5 20:46 54340000.vic -> ../../devices/50000000.host1x/54340000.vic/devfreq/54340000.vic
 
Here is a link https://www.jetsonhacks.com/2018/10/07/nvpmodel-nvidia-jetson-agx-xavier-developer-kit/
 
which decribes nvpmodel and jetson_clocks.sh (which was renamed to jetson_clocks, meanwhile)
Interesting of the link above is, that the performance model can be set nvpmodel.
If you take the nvpmodel program, the jetson_clocks script from nvidia ubuntu 18.04 and if you uncomment line 391 of jetson_clocks
    do_hotplug
    do_clusterswitch
    do_cpu
    #do_gpu <--- commented out at line 391
    do_emc
    do_fan
    do_nvpmodel
then the script seems to work.
Additionally  https://github.com/Pyrestone/jetson-fan-ctl should work then too.
 
I will try it and i report it to you.

My fan is really quiet with 255. I think it depends on the fan if it is too loud or not.

The temperature of my jetson nano is under 40 degrees too.
 
In tried https://github.com/Pyrestone/jetson-fan-ctl, but it works with ubuntu from nvidia only.
I tried nvidia ubuntu and the fan works really quiet and it sets the fan speed dynamically.
It uses /usr/bin/jetson_clocks which produces an error on armbian but the fan starts working. Maybe, on nvidia ubuntu are some defines set which are not available on armbian.
 
/usr/bin/jetson_clocks is used by https://github.com/Pyrestone/jetson-fan-ctl, too.
 
The do_gpu() function of /usr/bin/jetson_clocks produces an error:
Maybe, there are some environment variables about GPU not set in this section: /usr/bin/jetson_clocks line 242...
 
    for devfreq in /sys/class/devfreq/*; do
        name=$(tr -d '\0' <${devfreq}/device/of_node/name)
        if [[ "${name}" == "gv11b" || "${name}" == "gp10b" || "${name}" == "gpu" ]]; then
            GPU_MIN_FREQ="${devfreq}/min_freq"
            GPU_MAX_FREQ="${devfreq}/max_freq"
            GPU_CUR_FREQ="${devfreq}/cur_freq"
            GPU_RAIL_GATE="${devfreq}/device/railgate_enable"
            break
        fi
    done
    if [[ "${name}" == "" ]]; then
        echo "Error! Unknown GPU!"   <------------------- this error is shown on armbian
        exit 1
    fi