I suspect that if you just left it unplugged and let the water evaporate naturally it would be real quite. However it might reduce its output.
The addition of UV light to help purify the water is not a bad idea. There is a lot of research showing that UV is effective at killing many bugs. How about this gem. A device to stick onto your toilet bowl lid that to disinfect the bowl after each use? https://blueuvc.com/features/default.html
I actually helped the guy build his first prototypes.
He has a bunch of other UV projects, a toothbrush with a UV LED, floor sweeper, room air purifier, etc. or how about lights added to a hospital room the to disinfect before the next patient enters.
Back to your humidifier what about adding hydrogen peroxide (15 ounce to I gallon of water) this is supposed to help ward of colds and flu and in some cases prevent snoring.