What causes knocking in drain pipes when hot water drains?
materials tend to expand when they get hot, and that is true for
sewer pipe material as well. Whether it's cast iron or ABS, the
pipes will try to expand when they get hot and when they try to get
longer they'll slide across wood or other materials. The "knocking"
noise is the pipe fitting tightly against the wood and when the
pipe has expanded enough it moves suddently and for a short
distance, producing a "pop" type of sound. If you can expose the
pipe you can cut out the wood where it is tight and possibly put in
other material so that the pipe doesn't rub so hard.