Actually, a bunch of 40 mm fans would be good, where you need the air pressure, like when you're venting through a heatercore. The trick is that you have to build a shroud for each one
Otherwise you will get more flow out of bigger fans.
If you still want the silence, look into blowers, but they need to be fairly large to be effective.
I'll be using one with my heatercore. Car parts work best
