If the vacuum pumps are in different parts of the boat, it could just be that one area reverberates more than the other. When vacuum pumps were "stand alone" equipment (before SeaLand combined the vacuum tank and pump into a"vacuum generator"), mounting the pump on a thick piece of foam rubber helped a lot. I have no idea how to insulate a reverberating vacuum generator...not sure that even the $250 kit syjos suggested would help that.
--Peggie
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