I was having a similar problem with a whoosing noise while vacuuming and water draining when the pump was turned off. I was also getting air bubbles in the pump strainer while the pump was running. It turned out that the gasket around top of the the pump strainer was worn out. I replaced it and the problem went away.
Look for any air getting into the system, either through a bad gasket or a leaking pipe joint. Hopefully others will have some tips as well. Good luck!
Tad
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