When ever I run the hot water or pour hot water down my kitchen sink I get a knocking in the wall all the way to the second floor, it can also be heard outside. The same wall also knocks when the heater vent goes on. What can this be from? I climbed up on the roof ( which is pretty hard to get to in this spot) and there is some sort of pipe (with no cover) coming out of the roof. I can see steam coming out of it when the noise happens. The house is alittle over 2 years old. what do i need to do to fix this? I guess first I need to know what i am fixing....LOL

Thanks, Theresa