There is a surging sound coming from the plumbing between the water main and hot water tank in my house. It has a beat, with a period of about 1 second. It sounds sort of like the lines are "breathing." It sounds as if there's a bad toilet valve, but we've narrowed it down to the hot water system and nothing in the house which uses water is making any sound. We're not sure what causes it to start, but it lasts around ten minutes once it does. The sound stops once we turn off the water main, and when we turn off the valve leading into the water heater, but resumes when we turn it back on. There are no leaks that we can find (it's been happening for a couple weeks and there's no water damage in the ceiling) and the water heater doesn't make any sounds and doesn't vibrate. We can feel the beat in the pipe after the water main, and just coming into the water heater.
What could be causing this problem? We're fairly certain it's not the PRV, and it's not water hammer (no pipes are banging together, and there's no sharp crests in the sound).