When it rains a very loud knock or cracking sound comes from the vent pipe of the hot water tank( my guess). It should be noted that the hot water tank is in the basement and the pipe vents through two floors to the ceiling. The knocking is so loud I just can't believe it could be rain, dripping down the pipe. Also, the knocking is exactly 5-7 seconds apart. At first I thought maybe pipes expanding because of weather but I live in Vancouver, because and its about 14 Celsius outside.