The 9300 has two 60 mm Adda ball bearing fans connected to a small fan regulation board with three settings. It is mounted with a switch on the backside of the case. All the settings give to much Volt to the fans.
I tried giving the fans 5V, but there was still noise, even though the fan grills are removed, and the fans are connected to the case with rubber plugs.
The regulation board get 12V for the fans, and use resistors to slow down the fans. Giving the board 5V instead gives good result. Because I tried this first with a Papst fan which wouldn't move at the slowest setting, I removed the largest (R3) resistor, so I can't say if the Adda fans would go around with that one in place. However, with the R3 resistor removed (and the right foot soldered down where the left foot was), the fan moves so slow that I can see the fanblades as moving shadows rather than a shadowed circle.
There are still a little bit bearing noise, and I need to do a little bit better job of removing the fan grill, but I'm pretty much done with the case fans now. They make very litle sound, and at the slowest setting I'm not sure if I will be able to hear them at one meter even if I can remove all the other sounds in the case. And they do still push some air
