Hello I recently (few months ago) installed a tankless water heater following a failure of the old 55gal gas heater.
Performance has been fantastic, it actually resolved a number of problems, but I also noticed an increase in water hammer, most likely due to the fact that there is no tank to damped the water movement. I installed a couple of anti-hammer devices in strategic points and now that problem has been resolved too.
However, sometimes I still had noises coming out of the water heater itself, especially late at night. With the recent warm weather, I started leaving the windows open at night, and the other day I realized that those noises coincided exactly with my neighbor's sprinkler system becoming active. The noise repeated again 9 minutes later when the sprinkler changed zone.
The water meter of our houses shares the same location near the road, about 100ft from my house. I measured water pressure in my house while my neighbor's sprinklers are active and when they are not and it's similar at about 75PSI.
Two questions: should I suspect maybe his sprinklers, for some odd reasons, are connected to my own water lines? Pretty unlikely also given his house is much closer to the road than mine. Also, how do I fix the noise in the water heater? Note the noise comes from within the heater, not from the pipes.
As far as I know, my house does NOT have a pressure regulator, nor an expansion tank.
Also, I have never heard any noise from the heater during the day, and as I worked from home pretty often recently, I had plenty of times to stay in a very quiet house during the day.