Ran irrigation timer through a full cycle. At the end of the cycle when the timer valves off, hammering was occurring. Pinpointed the cause to the timer itself. Showed homeowner a thread on hammer arrestor that can threaded onto the hose bib to attempt to eliminate issue. Water pressure in home is 40psi. Water pressure isn’t an issue. If problem persists after arrester is installed, I would recommend replacing the undersized expansion tank on the water heater. Home currently has (2) 50 gallon heaters installed with a single 2 gallon expansion tank. Expansion tank should be minimum of 4 gallons and set air charge to 40psi. Homeowner will reach out to let us know if problem persists.