GrowCNanswered grow question 3 years ago You don't need to take in or exhaust air outside. It can help but it is not necessary. Most people use carbon filter on their fan's exhaust so the air can be dumped inside the home, or outside with greatly reduced odor. Not needing holes in the exterior for vents can help with stealth but you can also exhaust into an attic space or building's soffit.
As for a quiet fan I LOVE the AC Infinity Cloudline fans. I believe sold in Europe by MARS and under their name. The fans are the quietest I have seen and tried but keep in mind that moving air makes noise. If you want to keep it quiet oversize everything and run at reduced power. So, instead of a small fan running 100% get a larger one and run it at 50%. A VERY useful feature are the programmable controls of the AC infinity fans. You can set your temperature and humidity parameters and the controller will turn the fan on/off as needed.