I am starting my 5th year of running a 24 year old in-ground pool and am still learning - problems never end with this thing. I have a Nautilus NS-36 DE filter - and upon starting the pump this past week I noticed a "slushing" sound in the filter drum-barrel - towards the top near the pressure gauge. I took it apart this morning before work and all the grids and manifold looked secure, but noticed the strainer screen was missing from the air vent tube. I am guessing it fell or blew off at some stage of cleaning last season, and is possibly in the bottom of the barrel. Since I was late for work I had to reinstalled the drum and start the pump and get to work. My question is how critical is this strainer screen and what will happen if I don't get one back on soon?