You need to compare the airflow to the claimed noise level. There are some very quiet fans out there such as the Stealth series, but it because their maximum speed, and consequently maximum airflow, is less than similar form-factor fans. You'll get as good or better results by taking a good full-speed fan like some of the Panaflo's and just running it slower (with a fan controller or in-line resistor) and retain the option of bringing it back up to full speed (still not too noisy) if you ever need the airflow. SVC at least (and probably many other sites, I haven't checked) is good about listing noise levels and airflows. There is an occasional claim that seems out-of proportion and I tend to think "too good to be true" (I'm thinking particularly of the Thermaltake Thunderblade 120x25s claiming ~78 CFM @ 21 dBA - If anyone has one, and it is really anywhere near this good and stays that way at least for a while, let us know). In general if you look among the quieter fans of a given size, there doesn't seem to be any secret sauce, just different maximum airflows (and static pressures, if that matters in your application) and corresponding noise levels.
Based on my experience, you can't go wrong with the right panaflo at the right speed, and if you are in the valley, SVC does will-call.
nBd