Originally Posted by mitchgo
Any kind of possibility to access the supply plumbing elsewhere in the house such as the crawlspace or a different room that you are willing to cut out some drywall?
Nope. The knock best I can tell is right near the bathrooms, and the only access to that plumbing is through tile or the roof, both of which would cost a lot of money to fix. It's not a serious knock right now, sounds like the pipe might be moving somewhere in the neighborhood of 1/32-1/16", but nonetheless it is a weak point.