Could it be the particle sensor on the back? This has a small impeller that blows the air in front of the laser.

Same problem, the humming got worse and worse and now won't go away at all!

I own two 3Hs and have exactly the same hum or growl on one of the units as described. It is clearly audible and repeats about every 10 minutes when the unit is switched off. It is not my first Aur Purifier to have this problem. In 2020 it also happened with an Aur Purifier Pro. It was in use for about 1 year without any noise. And this noise was not present in the beginning with my 3H. When you sleep, it doesn't wake you up, but when you sit in the same room during the day, this humming or growling noise is very annoying. The noise goes away after about 20 to 22 seconds, but about 10 to 15 minutes later it starts again, over and over again. At first you think, ah the fan is revving up, but this is not a fan noise. It must come from the motor or the electronics. And once a device has this quirk, it doesn't go away. Either you still have a warranty, in which case I would bring it in, or you have to put up with it. In any case, it's very annoying.

The noise is also present in my case. When I switch the plug off and on again, the buzzing starts. But the unit does not run

With my device, I can't hear anything at all when it's switched off (whether via app or on the device).