That really depends on the AIO. Fractal's Celsius+ coolers have a mode where they run virtually silent, and Arctic's Liquid Freezer coolers run very quietly too.
There can be an arbitrarily large difference between “very quiet” and “silent”, or more precisely, between “still perceptible” and “not perceptible”. This depends on the environment (whether it’s so silent that you could hear a pin drop) and also on the individual hearing threshold and accommodation. AIOs are generally more perceptible than a silent build using fans.
Hm. Sure, silent and "very quiet" can be a different thing and you are right ofc about the subjectivity of perception. However:
> AIOs are generally more perceptible than a silent build using fans.
I'm honestly not so sure about that, but I thought the same before. But then I saw the charts of how well the AIOs can cool, which means the fans and pumps can be ramped down a lot. I know some models are loud, but know also from experience that not all are. Okay, an air cooler that cools mostly passive can't be beaten, but will also need air flow as soon as the TDP is higher - and on low TDPs an AIO can again be ramped down a lot.