I have a rackable HP 3000 XR UPS but it has continuously running fans that have a steady drone until there is a power outage when they kick up a level. My stack of HP DL380 systems drown out the UPS in normal operation. Only when the servers are shutdown can I hear the UPS fans. My hearing is not what it used to be plus I’m noise tolerant (spent some time as a roadie, used to maintain mainframe and minicomputer system hardware, etc.). So I’m not a good person to make a noise recommendation for a UPS. Obviously fanless is the way to go for minimal noise.
The specifications for my rackable UPS are: https://support.hpe.com/hpesc/public/docDisplay?docId=a00069340en_us&docLocale=en_US
The environmental specifications states noise as <46 dB at 1m (i.e. 3ft 3.3"). They don’t give the weighting on that (flat, A or C) but less than 46dB is in the range of a normal conversation at home. If you are sitting closer than 1 metre, obviously going to sound louder. My guess is that specification is only for the normal operating mode and not the noise level when supplying power during a blackout. I haven’t tried to measure the noise level so this is only a guess. God knows where the B&K meter is these days but I should be able to get some noise readings if needed. If you can obtain the environmental specifications for the UPS systems you are considering a lower noise level figure is better. Be careful comparing figures if different manufacturers use different noise weightings.
I’m still using fanless APC units bought at auction secondhand after Y2K plus an older unit. I’ve been very happy with their reliability. Just have to feed them new batteries when required. I have only had bad experiences with batteries that have a higher AHr rating than the original units. For example I replaced 18 AHr Yuasa batteries with cheaper 20AHr batteries as recommended by a local battery agent. They looked good on paper but didn’t have the same service life. It was as if the manufacturer had installed more lead in the batteries to achieve the higher rating at the cost of less liquid in the cells. It gives a higher rated capacity at the cost of service life. Whatever the actual reason I was not getting the same service life the experience means I now special order in Yuasa batteries. There should be other brands of sealed VRLA batteries like Enersys that may give good UPS service. My experience is limited to Yausa.