I had a thumping issue when operating my water pump. Since my thumping occurred when running either the hot or cold I knew it had to be close to the pump so that narrowed my initial search.
I eventually traced it to a water line leading from the pump that ran into the coach. I couldn't get my head into the area behind the Kant-Leak Panel so I used my cellphone to take several pictures and found a strap that had come loose. This allowed the line to thump against the top of the gray water tank.
Once I found the issue, I was able to get my hand behind this panel and used a "tie" to secure water line...lifting it off the gray water tank. Problem solved.
My guess is your noise is caused by something similar. Probably won't cause a problem but is kind of annoying. Good luck.
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