If the noise is in the drain, then water hammer arrestors or pressure reducers on the supply line will not change that. It probably relates to the type of pipe used and how it was installed.
I think we would need to know more about the installation. One drastic measure is to install a sound wall. Frame a new wall of 2x4 on their side, not touching the old wall, and spaced about 2" out. Fill all the cavities with fibreglass, then put up a layer of 1/2 soundboard, and then 1/2" drywall.
This is drastic, and you will loose about 4" from the room size. I would not go there until all the facts were known, and even then only if the noise where such a huge issue for you that it is worth the effort.