Does the well cap have a top and bottom held together with a gasket in the middle and bolts keeping the two halves together. Normally that is what you would need to have a sanitary cap on the well head. As for that sucking sound, the well should be allowed to suck in air as the pump runs so the well can't collapse. And the opposite as the well refills, you need a place for all that air to release to so it doesn't build pressure. Now I've seen wells that weren't set up like this, but I think that most should be.