There is a strange ticking sound coming from inside the wall area near the ceiling just to the right of my kitchen sink after we run the water. I’m pretty sure that this location corresponds to the location of a vent pipe on the roof. 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. Any ideas what this is and how I can fix it? Thanks.