I know this is like a month later but I ran into your post and well, im looking for ways to silence this beast. It really does move lots of air like everyone says. Yes it is pretty quiet when on low BUT only if there is no load (no duct work). Currently i have mine setup with no ducts on the out end (the rectangular port). As soon as I apply the duct to the intake end, the velocity is raised almost as if I turned it up to the highest setting and it becomes loud. The fan works much harder this way and I believe this is where the noise comes from.
Mine is the new model (4900)
2 things I want to try to remedy the noise are:
1) upgrade my 4" intake duct to a 5" and do away with the 5 - 4 reducer allowing for much more air to the fan.
2) attach a duct to the exhaust port and move it further away from the box.
Also i want to remove the black grid on the exhaust which i suspect makes that windy sound.
Im assuming you've already made your fan selection and have everything running seeing how its a month later but im curious what you chose to go with?