Sort of the bible for CFS500 is CWO4Gunner's review and modification thread. I read a bit of it and toward the end there's a suggestion to put the filter on a timer for a periodic shutdown to allow air that accumulates from cavitation to escape. See page 25, post #248. Maybe that's got something to do with your issue?... CFS500 is working great except getting air in from somewhere, which makes it a little noisier than silent. Suggestions, helpful hints or ideas are all greatly appreciated!