I researched those the other day and went with the 120mm Scythe S-FLEX SFF21D, its only 8.7dB!!! and 800rpm IIRC, plus they are extremely reliable with the Sony mfg. liquid filled bearings, supposedly good for 150k hours of operation. They also have an excellent reputation.
I dug around and found them for just over 11USD each, and with ordering 5 I got free shipping to boot!!
(needed some for a rack upgrade too)