Just got my new TD-silent 500 inline fan because my old fan was too loud. I have to say this fan is even more quiet than I thought. It sounds loud without anything attached, but when you hook up your carbon filter and some ducting, it's just a very light humming noise. I haven't tried this in real action yet, I'm just starting a grow soon, but it seems to move air very nicely. I've read some people say it doesn't suck so well through a filter, unless the filter is oversized. My already existing filter is a bit oversized and the fan sucks through it no problem. I thought I'd recommend this in case someone here is looking for a new quiet fan.
I have one with a phresh filter. Works great for me. One thing I did is use a on/off/on selector switch since it's a two speed fan. Using a potentiometer to control speed will increase noise. Mine always runs on high speed anyway. I have one of the smaller ones than yours I believe though. I opened my tent a few times and hit the Pax to try and see how quick it pulls air and its impressive. When closed I turn off the fan and after 30 seconds turn it on and the sides of the tent suck in. That's with both of the passive intakes on the bottom open.