Water hammer or loose pipe...
My house is brand new. We have a 50 gallon gas water heater by A.O. Smith. Has an explansion tank and a circulating pump installed.
I'm not sure if this has been happening all along but in the last week or so I've noticed that when the flow shuts off to the washing machine, or the toilet tank refills after flushing, and sometimes when a faucet is turned off, there's a loud thunking sound. Seems to be loudest in the wall behind the water heater. We are on city water and I'm not sure what the exact pressure is, but it's definitely pretty strong because I don't think I've lived anywhere with pressure this good.
A cursory Google search leads me to believe this is water hammer, but I've read some other stuff that talks about pipes not being strapped adequately. Since I'm hearing the sound in the wall, I'm wondering if that could be a possibility as well? Is there any way to know short of installing arrestors and seeing if that solves the problem? Additionally, do we install the arrestors on all of the valves that are exhibiting the problem, or is there some central location where one could be installed and control flow to all parts of the house?
Finally, does the expansion tank play any role in reducing hammer?