My UDM Pro SE was making a high-pitched squealing noise that was driving me crazy. I see posts saying that its fan ramps up when you install a hard drive, but I have no drive installed.
I wanted to confirm it was the UDM and not another device making the noise. Before unplugging it, I went to the front panel’s tiny screen and choose Settings, then Fan. It showed Auto, with a temperature of 54 degrees (Celsius, I assume). The RPM slider is about 1/3 from the left at 1200 rpm.
I touched the slider and dragged it to the left; the fan slowed and the noise stopped. When I released the slider, it bounced back to its Auto position.
I then dragged the slider all the way to the right. The fan sped way up, hitting 7200 rpm, and sounding more like a fan than a stuck pig. Again I released the slider, and it bounced back to 1/3 speed.
And the noise was gone!
Conclusion
Speeding up the fan for a few seconds blew out some dust or got the bearings turned around or something.
The fan may fail eventually, but hopefully this brief speed adjustment trick will solve things for awhile.