I’ve used the Nexus 120 SilentX fans for the last five years, but things always get better, so for quite a bit more money, you can get fans that are amazingly quiet and cool. SilentPCReview.com just updated their reviews and liked:
Noiseblocker M12-S1. It is expensive at $22 a pop, but unreal quiet spinning at only 870 rpm but cooling 25C at 12dbA (compared with the Nexus Real Silent Case Fan D12SL-12 cooling 24C at 14dBA, doesn’t seem like much but every 3dBA is doubling in noise).
Scythe Gentle Typhoon 120-12. This is sadly discontinued, but is an amazing 24C at just 12dBA so just slight worse than the Noiseblock but cheaper too.
There are some lower cost alternatives like the
Nexus Basic 120. This is the one I’ve been using and still quite decent and about $10 vs $22.