Even my plumber is stumped! My home is 60+ years old. The kitchen and one bath were replumbed 20+ years ago. The other two bathrooms are the problem. When I turn on the water, I get a very LOUD groan (not knocking)....the same when I turn it off. This happens almost every time, but not always. I do get a groan and high pitched noise in the shower if I turn on the hot water first. When I do the cold water and then turn on the hot water, it usually doesn't make much noise. It seems to be getting worse and is so volatile I'm afraid it's going to damage the pipes. Any ideas? Thanks.