I've been buying fans from the guys over at Sidewinder computers for years and currently using a couple of
Panaflo High Speed FBA09A12H1BX to cool down my fixture. Since noise is an issue your are looking a anything below 25dBa or lower and the quietest most CFM output you can find is the
Scythe SFF21D 33.5 CFM S-FLEX, the only downside to this 120mm (4.75in) fan is the cost of the hole saw to make a nice cutout and the wiring needed (though it's easy). If that's not an issue you won't find anything else out there that can match the 8.7dBA at that CFM rating.
Alternatively you can either use
Panaflo Ultra Quiet FBA09A12L1BX instead and use a lower voltage transformer (e.g.
6vdc instead of 12vdc) which reduces the fan's rpm resulting in a lower CFM and much lower dBA.
Keep in mind when using a transformer for these fans it must be
DC current and
NOT AC. Also, the
mA rating of the transformer should be equal or greater than the combined fans amp rating. (e.g. you need 2
Ultra Quiet FBA09A12L1BX then 110mA x 2 =
220mA, therefore a 220mA or higher transformer would be needed for proper operation.