Yeah, in my opinion those 140mm fans are getting loud at around 800 rpm when in a quiet room. I try to keep things below 800-900rpm for them.
Those 800-900rpm 140mm speeds I'm thinking about are for the worst case situation at full load. I try to set up the fan curves so that I get 400rpm when idle, and perhaps 600rpm for tasks that are making things a bit hot but where I'm not wearing headphones, then 800rpm for full load with a game with stressful graphics card use where I'm wearing headphones.
Meanwhile, I have 120mm fans that can be a good bit faster without being noticeable over the 140mm fans in the case. For example 1100rpm on "quiet" 120mm fan seems to compare well to 800rpm on a "quiet" 140mm fan.
When I was listing those speed settings I like for 140mm size, those were for the following fans that all use a similar shape for the fan blades: Thermalright TY-147, Noctua NF-A14, BeQuiet Silent Wings 3, Corsair ML-140. I've tried 3/4 of those and they all seem to get annoying for me personally at 800-900rpm. They do all work well with regards to performance, they are strong for the noise they make and seem to indeed be the best "quiet" fans you can get for getting fresh air into the case.
A thing that can sometimes happen if you are unlucky, there can be strange effects with air turbulence because of metal parts being in the way somewhere in front of the fan, or the dust filter interacting weirdly, or there can be vibrations getting amplified by the metal. You could try to experiment a bit, like check out the fan alone sitting on your mouse pad while connected to motherboard in the open PC case.