Once again I turn to the experts at CDC. The two pipes below drain the upstairs master bath and spare bath. They make a real annoying knocking sound ONLY when water is draining. They are PVC pipes that seem very secure. I've ran this by several buddies and have not got a definitive answer. I really don't want to call a plumber if this is an easy fix. Closest thing I've seen on YouTube is heat expanding and contracting in the pipes but it makes the noise when cold water is running as well. Handyman buddy says there is air in the pipe that can be taken out by some kind of respirator but the noise won't hurt anything. I tried to upload the noise but can't some reason.
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