Vent Tank

Are you hooked up to a city water connection? If so, do you have a valve you turn to fill the fresh tank? Is it fully shut off? Or the check valve in your pump has not sealed and is allowing water to back feed into the fresh tank. Turning off the city water supply and running the pump a few times may clear it, or there's a check valve kit available for the common RV pumps. An aftermarket check valve can also be installed in the pump line. The last, and most expensive, fix would be to replace the pump, although that's rarely necessary.