I had the plumber replace a water heater last week. It's a Rheem 50G power vent unit. Everything was working fine until today when there's hammering noise in our shower wall in the second floor and the water heater in the basement is shaking. What is the cause and what can I do until my plumber can come by?

I seem to be the only one in my home who hears it! I can hear what seems to be a constant trickling noise inside the pipes of the wall to the bathroom upstairs. Almost like the sound of water going down a rain gutter. It has been going on for a few weeks now. We recently replaced the full valve of the toilet which used to make a hissing sound. The hissing sound is gone and there are no leaks going into the toilet bowl. What could it possibly be? I checked under the townhome in the crawl space as well and could not see any water or hear it in the pipes down there. We are an outside unit. It is such a quiet constant noise that I am kept up, but my husband claims he cannot hear any noise. I would think with how much trickling it sounds like I would see some wet spots by now but I can't in the walls or the ceiling underneath. Please let me know what you think or advice you have. Thanks!