We just bought a house and have noticed a knocking/pinging noise from the wall when the gas heat turns on and after it turns off it is even louder. I'm prettty sure the vent is a direct run from the heater in the basement thru the wall in the family room (this wall backs to the attached garage) to the bedroom upstairs. From the bedroom it almost sounds like someone is throwing a ball against the side of the house. The noise is louder when the temperature is colder. I'm guessing it is the metal expanding and contracting... anyone know of a fix for this issue?
If you are speaking of a B-Vent flu pipe or also called a double wall chimney. Someone may have screwed the sections together. The inner and outer wall will expand and contract at different rates pulling the screw in and out of the inner wall. This type of pipe should not have screws in it.
Hopfully its something els simpler to correct.
Just a guess.