I have two upstairs toilets,in back-to-back bathrooms. I hear a loud water-rushing noise DOWNSTAIRS when either toilet is flushed. I am remodeling my house now and have opened up the wall to look at the toilet-drainline (is that called the sanitary line? Sewer line?). My plumber says it is properly installed and working well. But I hate the loud sound of rushing water! The plumber suggests extra insulation packed around the black pipe. I am happy to do that, but is there a better product to SOUNDPROOF the line?

Thanks,
Katya

Your drain is likely PVC rather than cast iron which was rather quiet. I would fill the wall cavity with rock wool or blown in insulation. Do not use vermiculite even if it is available in your area.

Black would probably be ABS. I had a similar noise, extended a short wall 2" and densely packed in fiberglass. Very little noise

I think I would put a stud about 1 1/2" away on each side of the pipe. Put up drywall on one side. Fill in between the pipe and studs and drywall with some expanding foam insulation. Fill the entire cavity. After the foam has dried, saw off the foam flush with the studs using a hand saw. That should minimize the vibration of the pipe, which is what noise is.