Air hammer
 

I've replaced the entire second floor copper pipes with CPVC. Since then, i'm noticing some air hammer that was not there before. I believe there might have been a homemade arrestor that I did not keep in the lines but that was nothing more than a 12 inch vertical pipe that was pinched at the top. That must have been filled with water since I had never drained the system to recharge it with air.

The knocking seems to be near water heater, also near where the main comes into the house. What is (and how many) the best place to install an air hammer arrestor. Excuse me if I'm using the wrong term. Also note, the only time I notice it is when any, not just upstairs, toilet is flushed.

One simple thing to try is to check that all water shut off valves under sinks, at toilets etc are full open. Since your issue is toilet related, check cold sides first.

That 12 vertical pipe is most likely an air filled water shock absorber...that is meant to prevent water hammering. But, you may also have loose pipes that are clanging. It's hard to say, since I'm not there to hear it. I know that I recently had a plumber replace a lot of my galvanized pipes with pex...and we had him leave them hanging looser than he would have so that I could accommodate a repair, and it caused a perceived water hammer. My plumber told me that the sound is most likely loose pipes...and as soon as he hangs them, that the problem should go away.

If not, I'm looking at installing an anti-water hammer device at my washing machine cold water inlet, since that's the culprit causing the banging.

Just one question, is there a valve, aka water spigot, near the 12 inch vertical pipe...one that you could use to drain it.

It seems that water will absorb air and sooner or later, you may have to drain the water out of the anti-water hammer device so that it will fill with air. If there isn't a valve near the water hammer device, the you can shut off the water to the house and drain the lines by opening an outside spigot lower than your anti-water hammer device. This should also allow the water to drain out of the water hammer device and allow air back into the pipes and the anti-water hammer device...and hopefully fix your issues.

Just some food for thought. Let us know what you did to fix it.