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When I used the plumbing as a ground for my ham station there was random scratching interference which correlated with the a/c running or someone walking on the floor above. It turned out to be that a water pipe was touching a heating duct. I “redressed” the pipe and all was well. I have since redone this with a separate ground wire to the electrical stake.
This is just a thought for your situation in that maybe something has changed in the plumbing. I would follow Brian’s suggestion first but thought I would offer this longshot idea.