Originally Posted by hduff
The wall-mounted exhaust fan is belt-driven and is the quietest of the fans.
From what I recall, the fan noise (that helicopter-like noise) is from the air passing over the blade tips.
That's correct, and the round tip shaped blades were developed to quiet down the fan noise. They still move air well, but cut down the noise factor. Try to find fans with the rounded tips on the blades, and they'll be quieter. Grainger has a pretty good variety of fan styles. You'll notice anything labeled "quiet" has rounded tip blades.
Air Circulators - Circulating Fans - Grainger Industrial Supply