Soundproofing Air Return Plenum. I have 2 air return plenums in my basement and they echo and vibrate something aweful. What can i do to sound proof them or at least stop the echo/vibration. I'll be decoupling the ceiling so that'll help but still. Also, they're nailed flush to the joists but is it ok to seal them to the joists as well so there's no leaks whatsoever.
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