Hey Jeffrey,
possibly bearing failure inside the fan.
That 40 mm fan is really not nice to your ears.
Could it be that your machine gets too hot, maybe because of too much wood dust inside? Cooling is already not best solved in this controller. You could try to run it open and simply disconnect the small fan (it’s a connector on the AVR mainboard), I have it this way (fanless) since one year, it is on all the time:
and for more protection against dust, protect it from dust like shown here.
Warning: Cleaning pcbs with compressed air can destroy the controller. Better keep wood dust outside.
I plan to nibble a 120 mm hole into the top lid of the case and to put a 120 mm Noctua fan into it, that blows upwards, and of course put everyting into a cabinet with a silent 120 mm fan and a filter like in the link above.
Or do you mean the 60 mm fan inside the Meanwell LRS-350?
I don’t think that I ever heard that one run at all. The noise could be the same cause, too much heat inside due to wood dust, which triggers an algorithm that lets it run fast (datasheet says “RTH3≧50°C FAN ON, ≦40°C FAN OFF”), or bearing is failing. It’s possible to replace that fan too, should the bearing fail, if you think you are able to open the power supply.
But always think of the fact that wood dust is a big enemy of electronics, so the cause could be simply the dust. Wood dust is a heat insulator and prevents electronic components from releasing their heat into the environment, even when a fan is blowing!