Nexus makes the quietest fans. I have a 92mm one on my Thermalright heatsink and I can barely hear it. It replaced a SilenX fan of the same size which was noticeably louder, which despite being rated at 14dba was probably more like 21.
<A HREF="http://www.us.ncix.com/search/index.php?Manufacturer=Nexus Technology" target="_new">http://www.us.ncix.com/search/index.php?Manufacturer=Nexus Technology</A>
<A HREF="http://www.silentpcreview.com/" target="_new">http://www.silentpcreview.com/</A>