I'm loving your build - a fan speed controller can do a lot to tone down fan noise. I know you're tight for space, but enclosing the fan in a box filled with some sort of insulation (or using that sound-deadening board material from the Depot) can help too, AND suspending that box with bungees or something elastic to keep any vibrations from transferring from the fan to the box might be worth considering. Also, don't know if you hooked the fan directly up to that duct elbow, but if you did and there's any vibrations from the fan, they might transfer to the elbow and make some noise that way. Hope this makes sense.