Who's responsible for noise in wall of coop?

There is a banging noise in the wall of my coop apt bathroom, day and night, when I hear flushing up or downstairs and when there is no flushing or water running sound. The mgt office tells the on-site sup't & he says he & a plumber can't figure it out & will do nothing else to find source of problem. its driving me up a wall. Someone--help