We got the 4" one off Ebay about a week ago, the filters in the room itself and its actually the loudest part, similar to the ballast (i want a digital sooo bad
). Outside the room it isnt heard. We hung the fan under our house from some beams and the only faint sound you can hear outside is the air flow, which im thinking may be quieter on a 6" vent. But its not very loud at all. Sorta sounds like the dishwasher is running. My hubby will be building a box around it today and from what ive read that should cut out the faint air flow sound completely. I think its a wise investment but i must say with our Bubblicious atleast you cant let it go off, the plants are sooo strong as soon as the fan goes off its minutes before i smell it. But thats fixed with a dimmer to slow the fan so it can be run (we dont need it wide open) all day.