I bought a house built in 1990 which has plastic drain lines. The bathroom drain line runs across the width of the house in the joists below the second floor. The horizontal run is about 35 feet. When someone takes a hot shower, the drain pipe makes a loud ticking as I would expect from thermal expansion. It is pretty objectionable. If I were to try to do something about it, I am guessing to expose it at the side wall where it makes the turn to go down beside the first floor wall is the most likely place it is under compression. If I can relieve the cramping that is where I am likely to find it. Right? The ticking eventually ends if more showers are taken, but it persists for minutes with the first one. Where this pipe leaves the side wall and enters the unfinished basement, and becomes visible, it is 4" schedule 40 pipe. It should be suspended with metal strapping up in the joist space, shouldn't it, with a gentle grade of about 1" per 10'. The other upstairs bathroom also does the same, but much less loudly. Unfortunately, this house has crown molding on the first floor, where I think I need to access the drain pipes. That means it will be considerably more work to repair any holes I create.