Have some tap on it and go were you think it is on the other side of the wall.you said there is a fire panel
On the garage wall, is there a sprinkler drain or a back flow device with a air gap drain on it going thru the wall
I would go check the Temperature Pressure Valve on the water heater. All the hammering may have effected it.
And, my solution to the water hammer would be the installation of a hammer arrestor some where in the line to the solenoid valve. The pvc is pretty springy, and the opening/closing speed of the solenoid valves on irrigation systems can initiate a water hammer which reflects up and down the line from the backflow valve to the solenoid valves.
My folks house in Vegas had a forty foot run from the back-flow valve to the sprinkler manifold. The first thing on the cold feed into the house was teh downstairs half bath. That toilet would just eat the fill valves. I was up watching TV at two in the morning, while the sprinlers were running, and on all of the zones the pressure surge that came back up the line when the solenoids closed was causing a pressure spike which was overpressurizing the toilet valve. And two of the 6 zones had slight water hammers when running.
Dad installed a surge arrestor at the end of the sprinkler manifold, and the problems all went away for twenty years until the surge arrestor failed.
looks like a pop moff to a water heater
check the T&P valve
Water heater is at diagonal opposite front of garage. I looked up what a T&P valve is, opened it, and saw that it drains out the opposite wall next to the water heater, but same type of drip spout. Just inside garage wall, right above the drip spout (by the water pressure damper) with the 2-cup a day drip, is the fire sprinklers control panel. Seems like a good bet that the drip is coming from there. See picture, chest high on inside garage wall. House/sprinklers are 23 years old. I'll try to find someone to look at the fire sprinklers. Thanks for your time.
The drip spout can only be from the fire sprinkler (FS) system, it's on the inside of the garage wall right where the FS system is, nothing else in anywhere in the vicinity. It appears that the hammering damaged one of the valves in the FS system. I'm having a fire sprinkler company come. They said that they cut into the drywall to get at the valves (2). Valve cost estimate, about $350.
Update - also replaced fire sprinkler pressure & drain valves, and rebuilt both irrigation system valves - still hammering.
We have a water hammering (noisy water pipes shuddering) issue when the irrigation system is running. So far we have had (1) the house water pressure reducing valve and irrigation backflow prevention unit replaced with quality Zurn Wilkins brand parts, (2) both the pressure and drain valves replaced on the fire sprinkler system, and (3) both irrigation valves rebuilt (new diaphragms & solenoids). We have irrigation drippers only, no sprinklers. The problem still persists.
I'll pass on to the plumber, Geofd's comment:
"Yes put a spring loaded check valve within 1 ft of the street side of the device".
And I'll also mention to him the "water hammer arrestor" comment of marlonc497.