I was thinking of mounting the fan in an insulated wooden box. Right next to the fan, I could put fiberglass insulation up against walls made of thick plywood or particle board.

The fan noise would be absorbed by the insulation, hit the heavy wall, and reflect back through the insulation (thus absorbing more of the sound energy).

I noticed that photos and videos of grow rooms sometimes show a fan mounted in a box, or someone even buying a fan already in such a box. Do you see those around the USA, or are these boxes more commonly found in shops outside the US?

What danger is there for the fiberglass insulation igniting and causing a fire? Seems like foam rubber or styro-foam would be MUCH more flammable.

I've also been thinking about a duct muffler. What is inside of duct mufflers, and how can I build my own? I'm guessing it's just a baffle of insulation, but it might just be a pass-through system for all I know.