While checking for leaks behind my water inlet panel last week, I inadvertently moved the "city water - tank fill" lever off of the "city water" stop and the pump did just as you described. Make sure the valve is moved all the way to the "city water" stop. If your valve is in the proper position, then you have a small leak somewhere.
It could also be the spring loaded check valve leaking in the water inlet connector.
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Fred Kennerson
2003 Dolphin L/X 6320
Yucaipa, Ca
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