Very few modern DSOs are fanless, some certainly could be in terms of power consumption, but are built to be inexpensive so spending for the big heatsink required wasn't worth it in the design...
As mentioned, though, R&S scopes are some of the quieter ones, but it's probably looking up model to model in other options. My experience has been that Siglent is a bit quieter than average than Rigol.... but honestly, my advice would be to just buy the scope with the features/price you want, then if it turns out to be loud and in otherwise good shape, crack it open and replace the fan. You can't expect noisy to no noise, but you can expect a notable reduction, so if you end up with one that's a little too noisy, it would be an easy fix.
That's me, though, I don't really have an objection to sticking my hands in test equipment when I've already verified its condition. I swapped the fan and added some heatsinks to my DS4024 that was a bit loud, and it's much better now (though certainly not silent, it needs a fair bit of airflow).