Be careful when buying your chosen fan. Many sellers on say Amazon, dont publish the db levels.
Some of the very quiet fans are down to 12Ddb. If you can find any at 25 db, or lower, they'd be fine. The sound of a person breathing at 1 metre in a quiet room, no other noise, is about 25db, as long as they're not asthmatic.
I think 24v 60x60x20 and quiet, might be tricky to find.