I cannot get to your fan link. Could you tell me brand / model?
I have been doing some research and the quietest fans I have found are, in the 115V, 120x38mm, around 50CFM
ORION OA109AP-11-3TB 25dB 1600RPM Ball 51CFM (source has no stock : ( ~$15)
PAPST/EBM 4840N 26dB 1650PRM sleve? 50CFM
PAPST/EBM 4800Z 28dB ? ? ?
USTF USTF129381152T 30dB 1950RPM Ball 76CFM (at Frys <$15)
The PAPST are expensive, around $40.