In our HT room... you can hear a pin drop on the carpet. Well... maybe I couldn't but Superman could.
Generally you can mod those fans with quieter ones with no adverse effects. I wanna say I've read something on the Behringer EP2500 I have, where you can rewire it to slow it down. Mabye it was at the CULT where I read it, but a lot of people have done it and again, absolutely no adverse effects that I've heard of anyway.