Recently I got hold of a Fortinet Lab FG-100D. The Fan in this unit is around 50db and not suitable for a lounge. The device is not licensed and out of support so I could ‘tweak’ it!

The quietest 4-Pin ( sending feedback back to to the Fortinet of fan speed so it can adjust ) 40mmx 20mm fan I could find was a NF-A4x20 with 15db of noise , however much-reduced airflow!

The NF-A4x20 comes with a different fan plug then the Fortinet Socket. The connect should match the other size , however a little pressure helps the fortinet socket accept this! 

The good thing about this fan is it also comes with a LNA ( Low noise adapter ) a cable that drops down the (voltage) speed of the fan. The 100D also has two power fan points. I was able to put the LNA on the original fan, then readjust the fans like below. I ran both for a week however sometimes the old fan would whir up, so I ended up disconnecting the original and left the NF-A4x20 and it has been stable!