After I shut off my taps air is getting into my fresh water system. This causes the pump to come on after a while. I cannot find any leaks from the pipework. So I checked the pump and can hear a slight noise like air passing through a small gap. Also I can see air appearing in the in-line filter, through the transparent casing. What do I do? Is it likely to be a waterpump fault? Why don't I see water escaping if there is air getting in?