Barbara in Saskatoon is complaining about the noise in the plumbing stack every time someone flushes the toilet in the bathroom upstairs.
In the old days that noise of falling water was muffled by the fact that the drain stack was made out of very heavy cast iron pipe. In the best of all worlds, even with our light weight plastic drain pipes, the water from above will swirl around the outside of the pipe and not make much noise. Some pipe layouts, even some minor clogs in the pipe, can cause the water to fall straight down to the bottom, making a lot of noise. You could snake out the connection from the offending bathroom to the drain stack, and occasionally that will solve the problem.
When nothing else works, and you are not about to try re-installing the drains, you can open up the wall and wrap the pipe in fiberglass, as you see in the photo. This will certainly muffle the sound. At the same time seal off any air leaks to that plumbing raceway as most sound travels through air paths. Insulation and air sealing will help a lot.