Hoagtech`s right, attic is a bad choice if temps get bad in the summer. Could lead to motor failure(my can fan has a thermal sensor that will turn itself off if its too hot) or start a fire.
Dont wory about the silencer unless the actual sound coming out of the exhaust duct with the air is too much noise, I can only feel air and no sound out of my plastic insulated 25ft duct.
Try the speed controller first, then if its still too much, then build a small box with sound insulation foam inside to cover the motor. If still too much then buy CAN FAN!
Dont forget to use foam liners on everything on your shelfing and such to reduce vibrations
Look:
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