As the efficiency is low (around 88%), all units have quite a bit of heat coming out of them. Fans are used to cool things down. But in general audible noise isn't an issue. Most of the time you simply hardwire the UPS into the group, keeping all the noise fi. in the basement. Get expert help for this kind of thing.
RT series start at 1000Va, around €719 excl. VAT. Note that at 700W output, it isn't powerful enough to feed an entire group. I'd advise to get a separate group for your audio gear if you really want to move into this path.
And please note investing this money into better equipment (which has a more stable power supply anyway) is IMO a much better idea.
I installed a APC UPS in my system. I can't remmeber right now what the model# is but its a fairly hefty one - came froma server rack and uses 4 12V SLA batteries. Weighs a ton.
There is a slight hum from the unit in use and its pretty noisy when it kicks in - loud click and the fan runs. You couldn't use it in the same room as a power source I think.
I installed it because I wanted the over and under voltage protection - which it does well enough apart from the noise. But I think it made a slight difference to the sound. But it definitely helped the picture on my TV.
Don't know if I would pay much money for one - but it was worthwhile to buy replacement batteries.
Definitely avoid the cheapo stand-by pieces of junk. They'll do more harm than good. Even the online models may not be so great when they're running on battery, cos they usually have cheapish transformers which output only simulated sine wave AC... closer to square... isn't great for PCs, and I don't imagine it'd help sound.