if you get a rheostat and hook it up to your 6" vortex its going to buzz a little bit when you turn the speed down. on full blast that fan is going to be loud even with insulated ducting and even at a little more than half speed its going to make some noise too. itll sound like one of your air conditioning vents does when the air is blowing out except itll be probably 2-3 times louder. hope that gives you some kind of idea.

check out harbor freight. they have lots of cheap supplies (made in china of course unfortunately). You can pick up a rheostat there for like 12 bucks (its the same made in china one hydro stores sell except its not marked up to 40 bucks). They have all kinds of neat things like machetes for when your in the deep woods scouting out your next outdoor patch or eye loupes for examining the crops and even digital scales on the cheap.