[GMGQ]

I would advise against putting any fibrous, insulation type stuff inside your cabinet. It will get humid and damp in there, and the material may degrade and fall into your sump, etc. I originally had some sound proofing foam around the back of my CL pump, and after a while it got encrusted with salt creep and it was just a mess, for very little gain.

I dont think lining your cabinet is the way to insulate sound. You want to leave the cabinet breathable, so that the humidity isnt enclosed inside.

Just take a step back and see what is actually causing the noise. Is it the trickle of water? The return pump humming against the bottom of the sump? (tip: get a mouse pad from the dollar store and put it under the pump). Etc.

For me, my main noise-maker is my closed loop pump, which is inside the cabinet. The noise is from the the pump whirring against the cabinet, which acts like an amplifier for the sound. If I held the pump in my hand (basically suspending it), then there's virtually no sound. So I screwed in a couple of hooks and I took some larger/longer zap straps and looped them around the pump and hung it off the hooks. The next problem was the spa-flex tubing that was touching the back of the cabinet where I had drilled holes for it to go through. Again, the vibrations from the tubing were being amplified by the back of the cabinet. I lessened the sound by putting some foam around the spa-flex, so that it doesnt directly touch the wood of the cabinet. I used that grey insulation tubing you can get at home depot (to wrap around hot water lines).

A friend of mine is always amazed at how quiet my tank is compared to his.