I can also recommend the TY-140/141 - pretty silent. they run from about 550rpm to 700rpm barely hearing them; they become more insistent up to about 900rpm - but it's not disturbing. higher than that, to about 1300,they are audible.
Of course, it also depends on background noise, other fans, etc and how sensitive your ears are
I used them as cooler push/pull, and now as top exhausts. You should also take into consideration that they have 120mm mounting holes, and are 140x160mm - so they might not fit everywhere.