I use strips of rubber mats to absorb the reverberations. These are nearly silent and running on floating 3/4” ply so they would be super loud otherwise. Hope that helps. I have a about 60’ liniar feet left. LMK if you can’t find anything else and I’ll find a way to get you some.
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Rice was a NO - GO.... I was thinking about hanging the pump from Bungees inside the stand- lol... I read a thread online somewhere that said if you place the pumps on the ground and not in the stand - they are very quiet. My Intention is to go home after work tonight and relocate the pump outside the stand (behind it) on the floor and see if that helps. I have about a foot between the stand and the back wall. Still not running it as its crazy loud!
As for your offer of rubber mat - If nothing else works , I will happily buy it from you and pay shipping to see if it helps.
PPS - Posting a build thread (all pics) right now. I will post a link when done =)