In monitoring the unit more it looks like the sounds is more pronounced when the fan is working harder.
I didnt notice this at home as I have the box fan blowing on the units.
My boss has let me put one in the office with the promise of allowing more if they are quiet enough. Hence, why I don't want to put in a box fan
I'll probably be upgrading the fan to see about keeping it working as quietly as possible.
In my experience the outside fan blowing toward the intake makes the whole setup quieter, and with that the internal fan works at less rpm and does not "whine" because it does not accelerate to higher rpm. Alternatively, people had attached a "pull" fan in front; just make sure the the airflow is directed to the outside with this fan. This setup did not work for me at all because temperatures inside the miner rose to 57-58oC, but perhaps I did not have patience to tinker with it more.
So, my personal preference is the regular fan blowing toward the intake. However, you know this already since you have this setup at home.