My old UPS, which was a cheap-but-evidently-not-cheap-enough no-brand one that made an annoying hum, recently became an Interruptible Power Supply, possibly due to battery failure or general patheticness.
So I've just bought a new one from UPS market leaders APC, and it's a great improvement. Virtually silent apart from a faint high-pitched eeeeeee that could just be my tinnitus returning. Even with the PC running my office is now eerily quiet.
Weirdly, though, among the various multilingual safety alerts etc. that filled the box, I found this: