Help, we have lived in this home for about 4 years. It was new construction. Recently we have started having a loud noise when you flush the toilet in just one bathroom. The noise actually fades away after the toilet has completed the flushing action. It does not sound like a pipe it is almost like a loud moan, the noise seems to be at its worst in the middle of the night or if no one has used that bathroom during the day. I'm thinking that it may be air in the pipes but that is as far as I can go. I do not have a clue. Do I need to hire a plumber? Thanks for any assistance any one can provide