seperate exhaust and fresh air intake. othwise how do you controll temps, and get fresh air into the grow area. im using an 8" 500cfm booster fan for my fresh air intakes in both tents. its probably only 150cfm, but i only need fresh air from it, nothing more.
Would say it'd be a very bad idea in terms of temperature control, unless you're also introducing a few other things into the "closed" circuit - used loosely. As is, and without anything else.. am guessing that it would drive up the temperature to a point that your plants will not like it at all.
i suspend my inline fan with giant cable ties coming from an eye bolt in the ceiling its pretty quiet that way. i seen someone make a soundproof box for it on the DIY somewhere but not sure how that worked out for them.
i hung both my 6" 440 cfm fans but those thick rubber bungee cords on a hoook in the basement ceiling joists, then i put a 36" piece of 6" duct on the air intake side. 2 fans running 100% i can barely make out the hum in my bedroom. and i have excellent hearing.
My tent is in my office. I also added a muffler. I can hear the fan but it really becomes white noise that I don't even notice. I am able to use my speakerphone all day without people hearing a wind tunnel.