Our downstairs shower room has a toilet, sink and shower. Recently it started giving gurgling noise from the draining hole of the shower. It also gurgles when toilet is flushed. The toilet when flushed, it fills water, and waits for a few seconds, and then drains. When it drains, the shower drain hole makes loud gurgling noise and also the drain hole cover flies out. What could be wrong, and how to deal with this problem?