Vent pipe bangs with hot water from sink only
I live on the top floor of a condo rowhouse with 3 units below. When running the hot water in the bathroom sink, after a few seconds the vent pipe behind the shower wall that goes up to the roof starts knocking/banging. It does not make noise with cold water from the sink, nor from the toilet. Running hot water in the shower does not make the noise. It only knocks from the sink hot water. On occasion, when units below run their water, the vent pipe bangs. We called a plumber out and they flushed the water line thinking that was the issue. It did not help. When on the roof you can clearly hear the noise from the vent pipe and lots of hot air comes up. We questioned whether the vent could have a clog but the plumber dismissed the idea. Can anyone help solve this mystery? Appreciate any feedback.
