The Orion fans similar to what we did the group buy on a while back are pretty darn quiet. When we did the group order, we ordered the higher RPM fans which move more air, so they are not super quiet, but still pretty quiet.
You can order a lower RPM Orion fan that you can hardly hear at all unless you are standing right next to it. Wayne (a.k.a. waynesreef) has four of these lower RPM Orion's on his tank and unless you put your ear about 3-4" away from it, you can't even hear them running.
The one's we got are a bit louder than that, but still very reasonable in my opinion.
What does anyone else think on those Orion's we got on the group order?