Question: Why are my pipes moaning? What should I do if my pipes moan?
Answer: If your pipes are moaning, the bracing of the pipes is likely to be the problem. The pipes may be running through small holes and rubbing as they expand (because of hot water making them expand), or they may be angled incorrectly.
If it's your water heater that is moaning and not the pipes in the wall, the pressure relief valve could be positioned wrong.
What You Can Do
Identify if it's the pipes moaning (inside the walls) or if it's your water heater pressure relief valve. If the issue is just the pipes, you can probably ignore it—there's no real harm being done—or re-anchor the pipes in question. Or you can re-anchor or realign the pipes, assuming you are comfortable performing basic construction work.
If it's your water heater, you need professional help. Your water heater pressure relief valve is dangerous to change yourself, so you will want to hire a plumber if you need to replace the pressure relief valve.
Learn about water heater pressure relief valves to understand why this is a problem that is dangerous to try to resolve on your own.
Read more about pipes making noise to understand what is happening and what you can do.