Occassionally I notice a strange ticking sound coming from inside the wall area near the ceiling just to the right of my kitchen sink. I’m pretty sure that this location corresponds to the location of a vent pipe on the roof. I’m pretty sure that there is not any copper water supply pipes in this area. Sometimes the ticking is quite rapid and other times it is very slow (once every 30 seconds or so). I’ve noticed that sometimes it starts quick and then slows down and eventually stops. I noticed it the other day when my dishwasher was running but I’ve run the dishwasher since then and there was no ticking sound that time. Any ideas would be appreciated. Thanks.