Hmmm. Decrease the lift? Cavitation? Does that mean that your pre-pump is sitting higher in the little holder? I think the gas level is about 1/4 in this tank and I am fairly sure I moved the pump down to the stops in that holder. Perhaps the tank getting full would make the difference. The pump came from FCPGroton and I have never had sound issues before making me think that I may have put the new bigger and flatter sock on in a funky position. I should have paid attention to how the previous sock was positioned. Perhaps the sock is resting on the circular partition in the bottom of the tank and sending the noise? Gotta pull it tomorrow. It was particularly tough putting it in so I think I messed something up. I also used the old rubber o-ring at the top and that may be the issue. Huh! Just remembered that. I will keep you posted.