So for the sake of testing, I tried just swapping the fan, without any additional thermal management complications. I ordered the first 50x50mm 24VDC quiet fan I could find:
As a reference I captured the sound level at the fan output with the original setup: around 76dB
While I was recording this, on a hunch I tried removing the fan from its housing while it was turning, and that’s when I realized that the horrible noise of the Huanyang VFD is not actually the fan’s ! It’s the sound of the air making its way through the (giant!) heatsink fins inside the VFD (that thing is massive).
For no good reason I decided to cut parts of the air intake grid underneath the VFD, to see if that helped (DON’T DO THIS, don’t insert your fingers, be safe, etc…)
and since I had bought a new fan and standalone testing seemed to show it was a bit quieter, I removed the stock one and installed it (soldered it in place)
Results are slightly disappointing, the sound is down to about 72dB (well, that’s still ~50% quieter)
EDIT: I forgot to mention also that just removing the fan cover improves the noise level.
I don’t see a good reason to keep that piece of plastic, so off it goes.
My next step will probably be to just add a resistor inthe circuit to drop the voltage so that the fan spins slower, I’m pretty sure that a lower air flow will be enough to keep that monster heatsink nice and cool.
Then I’ll probably buy one of those cheap thermal controllers I linked above too if only for the sake of trying them.