I have one pipe that drives us crazy. IT IS VERY LOUD - like a cannon going off. I've heard all the theories and nothing seems to work. So, how about a MUFFLER angle if the pipe is accessible. How about a plastic tube (slit obviously in the middle) slid around the pipe and filling it with some muffling material (would insulation material work ? Something that is not flammable) Maybe this would at least MUFFLE the sound. I am going to try it.