My recommendation would be to stand your filter up on end and run straight - on a shelf maybe up higher than the floor. (put supplemental circulation fans under it, so you capitalize on space) If you can't get the fan up out of the closet and into where it's venting (which I wouldn't recommend to anyone NOT running a Green Gator as you lose efficiency the farther down the duct your fan is from your filter), insulate the crap out of the lower portion, but keep your motor area free to keep from overheating. If it has to stay in the closet, you will still be able to dampen the sound by hanging it with some heavy duty bungie cords or mount eye bolts in it and use cable ties of some sort. I use ratchet straps. If you keep your ducting straight, you can run it at 25% or less, I'm betting - even with your fan on the lowest setting (and I would get a voltage controller...but don't skimp...fire, fire...danger, danger...). Now, if hanging is out, or proves too difficult, or you need to wait a little while with your seedlings going, get something like some rubber mounts for the way you have it hung now to go in between the fan bracket and studs - kind of like motor mounts. The Green Gator flows so much better, it will actually increase your dwell time, and scrub oh so nicely
I run both fans in here, and can't here them over my window unit. Bedroom is on other side of the wall, and my wife is ONE OF THOSE...lol. My fans are 440CFMs...I don't know how large yours is...but I bet it is probably better quality.