Found an absolutely silent micro fan system, $35

Newegg.com - Scythe S-FLEX SFF21D 120mm Case Fan - Case Fans

Power Adapter AC to 12V DC 100ma PS1201USA

33.5 CFM, 8 dB of noise. Compare that to the 60 dB you typically get with a 4" inline fan. I'll keep this one running 24/7 but only have my loud 4" inline during the light periods where its needed for venting hot air. When the lights are off (so heating isn't an issue), if you have any relatively small sized cab 33 CFM be adequate ventilation.

The fan was really the last non stealthy part of my setup. It was ok to have my fan off during the hours of possible people coming into my apartment (11am-5pm) which also happen to be the dark hours of my plant during vegetative. The problem came when I NEEDED that fan on at all times just to filter the air so it didn't make my entire apartment reek. With the fan on, well, having a humming box full of weed in my closet if there was an apartment showing wasn't a pleasant thought. But it was either that or an even more conspicuous weed smelling room.

This guy should do the trick, almost absolutely silent (about 3x more quiet than regular pc case fans) and will vent more than enough air to prevent smell in the final weeks of flowering. My setup is finally safe :)

Just thought you guys might be interested. Also only draws 1.2 W