I'll chime in on this one.
First of all, the noise comes from the fan blades striking the air, mostly. Some time back we had this discussion, and someone (dared to?!) tell me that I was wrong, until someone was kind enough to experiment with a fan. He recorded the noise, then clipped the fan blades and recorded it again. The noise was (mostly) gone.
Now, the noise is going to depend a lot on the fan blade geometry: too pitched, and they'll be noisier than hell, too flat, and they won't move enough air.
Someone understood this, and charted (from specs) all the fans he could find, into a nice cfm to noise ratio. The highest ratio unit is the most desirable, regardless of how much noise it makes at 12V, because it can be undervolted to an acceptable noise level, while delivering the highest airflow.
That being said... this chart only included axial fans. There are other types of fans, like the squirrel cage blower, which are (theoretically) much quieter than any axial fan...
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