We have a bedroom in a walkout basement and yesterday we noticed that the water could be heard clearly rushing through the pipes behind the wall every time a specific toilet is flushed or we use the kitchen sink/dishware/disposal (there is 90 degree pipe that the water is hitting on it's way out of the house). The noise is loud enough to wake someone up.

We had a plumber look at it and he recommended that we get additional insulation and double the dry wall to muffle the sound. He said we could replace with a cast iron pipe but that would not really make that much difference. We are thiking of getting some noise dampening pipe wrap and maybe putting in an additional sound barriers in the wall.

Has anyone tried that? Any opinion on whether it will work?

Don't rip out the cast iron, PVC would be even noisier. Faceless insulation the full thickness of the stud cavity and double drywall will do tons to cure this problem. How is it that you just noticed this, it has been happening since the first flush?

Just bought the house a few months ago and no one was sleeping in that room. Actually the plumbing is currently pvc. We are getting ready to replace with cast iron and then pack in the insulation.