How old is the pump? Did it freeze over the winter? I would check the parting line on the pump and look for white corrosion indicating a small leak over time. If it is leaking at the parting plane where the rubber membrane is sealed it will draw air in there and cause the effect that you are seeing. If you see no evidence of a leak there there,it may be a loose or cracked fitting in the supply line from your water tank. You can also disconnect the supply line from your FW tank and put a line to the pump from a 5 gallon pail if no air comes out it is your supply line if air still comes out it is your pump leaking.