Hey there!
I am new here, because I recently acquired a used Balance 1350 HW2. When I first flipped it on, I was instantly teleported to an airfield, where a plane started from within the fan outlets 
I am used to loud hardware in other contexts of course, such as PoE-Switches or other IT stuff, but was not expecting the 1HE router to be so loud.
My usecase is mainly aggregating WANs via SpeedFusion for live streaming and live events and I wanted to use it near the front of house or on the sidestage. But with those loud fans I really would rather not, unless I like being haunted by the sound crew or anyone else who is noise sensitive (like myself :D)
As it is an old device without any warranty left and I am confident I can open it up (and reassemble it) safely and in accordance with electrical regulations, I want to swap the fans for more silent ones. Now Iād like to ask the swarm intelligence for information about which fans I can use for that. 
I am based in Europe and know Noctua fans, but do not know a lot about issues like PWM and RPM-control and what I should focus on when changing the fans. I would not mind them to run on full speed, if they are silent, though. But would changing the PWM fans to some that are not controllable cause errors in the firmware or something? Does anyone have experience with that?
I would appreciate input on this matter!
Kind regards
Julian