Hot water pulsating

When I turned on the hot water, the water would pulsate as if the water had been turned off and air was in the line. This only happened when the hot water was on and I ran the water for about 15 minutes and it continued to do this. Any ideas.

Find any circulating pumps, pressure reducing valves, or filtration systems anywhere?

Problem at every fixture?

I found no valves, pumps or anything that would give me a clue. The only thing that I found that gave me any ideas was the piping coming off the water heater was flex copper and it had pretty good bend in it but it was not kinked. Yes the problem was at every fixture.

How about a loose washer in the valve feeding the water heater?
Roy Cooke

I have seen this once with a globe valve that was installed backwards. I know you said there were no valves, but it couldn’t hurt to look again.