Knocking sound in wall

[prosc]

We built a new upstairs bath. 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.

 

[lordofthepipes]

you can get a (knocking/ticking) sound from the hot water lines if the piping is strapped too tightly to wood, or if the piping is going thru a piece of wood and the hole is too small. when hot water runs thru piping it expands the piping and can make the knocking/ticking sound. this can also happen with pvc drain lines too. sound to me like it might be the drain. If this is the problem the only way i know to fix it is to open the drywall, find the culprit, and make the hole bigger/use different pipe supports.