Diabolical water hammer

Hi,

My house is two story, slab with copper piping. Every fixture/appliance in the house makes some noise when it shuts off, even sink faucets but solenoid based devices make a really loud bang. I often even hear noise when there is a constant low volume flow - sounds like a coffee maker percolating coming from the water heater area (water heater not heating at the time). Kind of like a click-click-click noise, very regular.

When I hold on to the cold water supply to the water heater I can feel the shock on the line when something turns off. Often it will echo several times over 2 secs or so. The sound seems to come from the water heater area and propagates through the home more acoustically than from a shock to the framing so I don't think it's a loose pipe.

I carefully checked my main water pressure and it's very stable at 50 psi.

All piping for my sprinkler system is outside of the house.

So, I installed a Oatey 1/2" arrestor at the water heater cold water supply. It didn't make any difference!

I have a toliet that is 10' from the water heater and when it shuts off I can clearly hear the noise (after installing supressor). You would think being so close it would at least help with that fixture.

Any ideas what's going on here?

Thanks

P.S. I asked about this on another forum and the "experts" there assumed I had a pressure regulator somewhere in the system which is causing a pressure rise in the system due to the water heater. I looked for any access panels in my garage and did not see any. I do not have an expansion tank for the water heater.

If you have a closed system you need an expansion tank but I don't think that's what is causing the hammer effect. Just because your toilet is close doesn't necessarily mean the piping is close and without bends. Can you get to the drop to the toilet? I would as an experiment put a hammer arrestor at the top of the drop.

Are your pipes secured to structure, almost sounds like besides the hammer you might have some loose pipes.

Ok, I'm an idiot.

There is a cold water shutoff to the water heater. I shut this off and ran sprinklers etc. and not a peep! I should have thought of this earlier.

My water heater is an A.O. Smith FGR, FSG, etc.

Any idea why the WH could make noise? I thought it's just a passive tank - no valves etc.

Thanks

P.S. I read some material on A.O. Smiths website and I also know that I have a lot of mineral build up since my water heater pops and gurgles when heating. I don't know if this is related to the water hammer but I'm going to de-lime it.