We built a new upstairs bath 8 months ago. When the hot water tap at the sink is opened we get a knocking sound from behind a wall across from the sink. There are no supply lines in that area, only the vent pipe. After closing the tap the sound continues for about 30 seconds. However if I flush the toilet which is located next to this wall, the noise stops. The plumber added an extra vent across the room for the washer. The supply lines are plastic. The plumber is one of the best in the area and I trust his work.