Hi, everyone, I have one water heating systerm on the basement and the first floor. Recently, I have heard the running water noise (it is really big) in the pipe at the first heating up, especially on the first floor. I have "purged" the system fllowing the steps in Youtube and didn't find lot air in the pipe. After that, the noise came back when the heating. I have check the pressure reading, which is normal in 12-15 PSI? Anyone can give some suggestion on this? I am wondering the expansion tank but don't know how to check~~~ Thanks in advanced.