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How about having the air inake at the bottom? Its kinda hard to visualise the setup you have explained, but I am guessing the noise is being caused by the air flowing through the tube, like wind in a tunnel? I usually have the carbon filter on the inside so that might change things. It would be cool to see some pictures.
here's a terrible drawing of what I'm trying to explain
i would put that carbon scrubber inside the cab with the squirl fan in the attic.....you can even make a box to go around the squirel fan to silent the operation of the fan......when i tried to cool some 1000 hps....i put the hydrofarm 460cfm in my attic inside of one of those a/c junction boxes....the ones that usually are found directly about the a/c unit to split the a/c ducting.....you can find these at any a/c store ive really never seen them at hd or loews.. enclose the fan inside the box....i use to hear a loud humm with the squirel fan in my attic....put a box around it couldnt even tell if it was on....i was constantly going up in the attic to see if the fan was running...
another thing......i might be mistaken but you are going to need more than 90 cfm in that room......with any type of 90 degree angles...i could be mistaken.....i would read rhizome thread about intake and exhaust sizes and fan.....its really good info......sorry about the double post...i really didnt read the 90 cfm.....you mignt want to try around 200 cfm...Correct me if im wrong
killa12345