this is the third time in a month the manhole has blocked. it took me at least 20 minutes to unblock every time using set of rods with plunger at the end. last time there was not any dirt in it, just clear rain water. when I unblock it, all I can see is a lot of trapped air coming out. does this mean it needs some sort venting, if so why now, it has been working like this since the house was built 1920.