DUNBAR
You need to trace that drain line to where it is in contact with a wood surface in the wall.
Thermal expansion of the plastic piping is “ticking” across the area where there is movement against the two objects.
The only way your going to find it is to start opening the wall where it is either braced, or passes through the joist area. Access directly behind shower is useless unless you can visibly see the piping touching the wood where it is in contact with.
When you do find it……you are going to have to cut out or chisel the wood out farther away from the piping to allow free movement for thermal expansion.
If your not willing to open the walls, then you will have to deal with the sound it makes.
