It is sometimes caused by draining your hot water tank through the low point drains instead of closing the bypass valves and draining it through the anode/drain port in the water heater. This causes bits of anode and/or calcium nodules to be left in the cold water lines when drained. When you de-winterize, incoming water can push that crud into your faucets (causing them to leak), toilet valve (not to flush), and the water pump check valve (preventing ot from closing).
To TRY to fix this:
Depressurize your lines and remove the city water hose.
Turn on the water pump and open a faucet.
Shut the faucet and turn off the pump.
Reattach the city water hose and see if you can hear air (or see water) coming out of the fresh tank overflow, still.
If that does not fix it you have two choices.
Replace the pump head or pump
Add an additional check valve on the pressure side (opposite the inlet filter) of the water pump.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW
I agree with the other folks, it is probably your check valve in your water pump. I had the same problem last week. I turned off my city water, and ran my water pump for about 10 minutes, cleared it up. However, I did buy an external check valve to put on if the problem reoccurs.