Hackaday just refers to another guys post, but they've earned, and deserve the love.I went back and double checked my 6450 non-poe in my office and I completely forgot to swap pins on the Sunon in this switch when I installed it. Whoops, no wonder it was completely silent. High temp over the last month was only 62°C so no harm done but I swapped pins so the fan will actually function.
Is there any way to address the pwm noise at low speed on these? I'm willing to cobble together some simple electronics inside the case to quiet things back down, the pwm noise is actually really irritating in a quiet room.
Short version, a low pass filter tuned to the pwm freq, ie a resistor and capacitor. You'll waste some power in the resistor but oh well.
However the fan itself is brushless so is already doing some 'pwm' of it's own to drive the coils, which is speed dependent and therefore isn't constant. That's a reason PWM to a fans power pin is a 'suboptimal solution' and 4 pin fans are a thing (4th pin being a command signal that the fan uses to adjust itself, rather than via incoming power manipulation).
Your incoming PWM of the power to the fan 'fights' with the frequency of the coils turning on and off in the brushless motor, and you get some 'beating' constructive/destructive interference. Basically, if the fan is in a position to require power to the one motor coil, and the incoming power PWM is 'on', all is good. If those are out of sync, no power is provided, fan slows down. The increased noise is probably times when the incoming pwm cycles in the middle of a coil activation, 'suddenly' providing or dropping power, jerking the motor/fan. Will it hurt the fan? probably not as much as eventual lubrication loss, and the fan probably has enough inertial and a low enough power/torque in the motor to smooth things out. It just ends up causing noise.
Trying to figure out how to balance the filter to the fan is probably a fools errand (given the fan has at least 2 speeds) but filtering the PWM should be easy. If you multimeter has a frequency counter feature (I think my $35 extech does) you can get the pwm freq from that, run a low pass filter calc on that, and size your cap/resistor pair. But the results to sound may vary and I'd be a bit surprised if there wasn't some filtering already on the mainboard. Hooking a oscilloscope to the fan header, it would be obvious if it was pure pwm or if there was some filtering, but I haven't had an o-scope to hand for a fair few years.