I'm afraid OP will have to find his own "comfort zone". No matter how quiet or loud it is to others or what decibel meter says, perception of loud/quiet is very individualized.
I have a certain condition that makes me extremely sensitive to sound. Also, I have a "dip" in sensitivity on one ear. To me, high pitch sound seem louder than lower pitch sound. (think mosquito at night!) In quiet place, even a little noise sound louder. Funny, I have a noise cancelling headset. It only cancels repetitive sound. Using it is sometimes worse from perceptive point of view as very little non repetitive noise comes right through. Sometimes 10 dollar ear muff from Home Depot works better.
Fans are cheap to replace. I've been going through a few that irritates me. That said, my Siglent is pretty quiet and barely audible. Since the fan is on back side of it, you can also be creative and try to remove noise that bounces off of a wall behind it. I am uploading a photo of part of my lab. On it, you'll see black and blue foam I bought from Amazon cheap. White bar across it is my lighting. Those foams are thin but effective in preventing sound bouncing.
Run higher voltage fan with lower voltage power is also possible. Often times, slowing it down is a good solution. Some computer type fans are amazingly quiet, yet enough air to be effective.