Finding a quiet one is unlikely as noise probably does not factor into the design of these things.
Another thing to consider is the plywood decking resonates considerably. Are you willing to go to extreme measures to sample various fans, isolating mounts etc?
I can give you a bit of insight from personal experience though. Having had my roof replaced about 2 yrs ago I went looking for a fan but not wanting to deal with the wiring and controllers etc. I went for a solar unit.
Prior to installing the fan you could not even go in the attic in the afternoon, the temps were unbearable (and I love the heat) now it is tolerable. Temp measurements were never taken, but I would guess that we were talking 120+F prior and 90-100F after.
The instructions called for southern placement of the solar panel but I opted for western exposure so it would run from mid afternoon to sunset.
The unit that was installed is not silent, more quiet than anything. Ambient noise easily drowns it out ( traffic, neighbors, birds ) when you do hear it, it is more of a hum that resonates through the wood structure of the roof.
I would definitely buy one again, no wiring to contend with, huge temp difference, roof should last longer, noise is rarely noticed ( sounds like a refrigerator humming away in the other room ), no increase to the utility bill.
Link to the one I used. http://www.homedepot.com/Building-Mater ... escription