On my 750D the fan settings are either HIGH or LOW. The LOW did not work. HIGH was too loud.
I tried some fixes but no joy. Since there was a lot of room behind it, I mounted a 110v computer "muffin fan" to the air outlet port normally used by the factory fan. Then ran that through a rheostat to control speed.
Now it's dialed in to run at the fastest speed with lowest noise. 750's case doesn't even get warm now.