Water hammer - faulty valve?

My house is about 9 years old, and in the last couple years, I have started hearing noises in the wall of one of my showers. Its on the second floor of my house, which is also where the plumber installed home made air chambers. The sounds in this wall typically occur when any hot water valve anywhere in the house in opened and then closed quickly. It sounds like rumbling in the wall, and I wonder if its the valve itself, because I made a small hole behind the baseboard and made sure the pipes are stable. I've tried draining and refilling the system to try and re-charge the chambers, but it didn't make a difference. The other reason why I wonder if its the valve itself, is because there is a specific position that I can turn the valve lever to, that will always cause violent water hammer. Maybe I should mention this is a pressure balance valve. So my question is, could it be the valve that needs to be replaced. Also, how do I get the pressure balance unit out of the valve body. I removed the handle, etc, and the retaining ring, but I can't get the balancing unti out. I tried so hard once before, that I broke the front face off it, and then pushed it back into the valve body, verified it still sealed and gave up. The noise is getting so frequent its time I try and resolve this once and for all. I've considered adding store bought water hammer arresters and decided not to, just in case its the valve that is the problem. Remember that any hot water valve in the house 2400 ft2 causes only this showers valve to act up. Lastly, its an American Standard Hampton Shower valve.
Any help would be appreciated.

Water hammer is caused by a rapidly closing valve, NOT by one partially closed. That is vibration and IS caused by the valve itself if it only happens with one or two valves. It is almost impossible to "recharge" air chambers once the air is absorbed, even if you leave the water turned off for weeks, and maybe months, and definitely not minutes or hours.

Hello, I am new to this forum and this is my first post. I was experiencing the same problem with a Delta 13/14 series bath shower valve that is maybe 3 years old. The knocking noise was made anytime hot or cold water was turned on or off or the toilet flushed. The noise was throughout the house and was driving me crazy. I had family members turning the water on and off while I had my ears to the wall trying to figure out what pipes were banging. After listening to all the walls where I knew the pipes were located I still could not pin point the source of knocking. I then searched the Internet and found this site. Doing some research I noted several members said replacing the pressure balance valve if the valve does not rattle or if there is junk inside. So I took out the valve and it rattled and then I opened the balancing valve and it was clean and the balancing piston was sliding back and forth just fine. I reassembled the valve and reinstalled it and it was still knocking. So I thought about the design of the valve and the theory behind it. My best guess is that the balancing valve piston must have worn down enough over the years after sliding back and forth so many time and that the valve piston must be sliding too far left and right and banging the valve body wall and making the noise. This is a simple design with only three likely point of failure in the balancing valve. Either the valve piston or the walls in which the piston slides are worn or the o-rings that seal the valve inside the housing have failed and are leaking when they should be water tight. These three components if working properly should dampen the sliding piston so that it does not bang the valve wall when it reaches the farthest most range of its travel. The only solution is to replace the valve cartridge. So I bought a new valve cartridge and the knocking is now gone. Just FYI, shaking the valve and hearing it rattle does not guarantee that the valve is good. It just confirms that the piston is free to move as it should. I disassembled the balancing valve and there was some minor wear, and the o-rings all looked fine. So it must have been that small amount of wear in the piston or valve body that is causing the dampening affect to fail and the valve piston to crash into the valve body. Thanks to all of the participants on this site that pointed me in the right direction. This was the first pressure balance valve installed in my house so this was a new trouble for me. Thanks.

The pressure balancing spool valve does not move back and forth normally. If only operates when that valve is turned on AND the pressure on one supply varies from the other (note, this is not just the volume of water flowing, it is the pressure behind it). Functionally, say your pipes are not sized properly...someone flushes the toilet, your cold water pressure drops...the spool valve slides one way, decreasing the hot water supply, keeping the mix coming out of the valve fairly constant so you don't get a hot surge because there's no longer any cold water coming in, or at least not as much.

On a valve that uses washers (verses, say a cartridge), if the washer is wearing out, as water flows, it can move under some circumstances, and cause the flow to momentarily stop, allowing the washer to fall back, then cause it to stop again. This typically is more like a constant bang, bang, bang, (machine-gun) verses a single thump. 1/4-turn ball valves don't have that issue, not do most cartridges.

Some valve designs will say that a hammer arrester is required. On those, you really do want to follow the manufacturer's instructions!

I experienced what sounds like the identical problem okarob did. When we would turn the cold water on or off suddenly in ANY faucet in the house or flush the toilet the we would hear about 4 fairly loud bangs that we heard through walls and up into the ceiling(the sound really traveled and the location could not be isolated without a visual inspection). I was concerned that the pipes might come loose at a joint or rub against a sharp edge and cause a leak. I had several guys out and the consensus was we all thought that somewhere a "fastener" holding the pipes had come loose. We ended up cutting a hole in the wall to try and locate the source and did not see anything that had come loose but the pipes from the shower/tub were not clamped to the studs and NEVER were. We could still not reach those pipes without cutting out a hole in another wall so the plumber suggested trying to replace the Delta cartridge(part number RP19804). He did and that RESOLVED our problem without having to tear out more wall and find a way to fasten the pipes from that shower/tub facet

FWIW, some valves actually say to install hammer arrestors during the installation because they were engineered to rapidly shut the water off. Some toilet valves do this, and things like washing machines, ice makers, dishwashers, etc., that have solenoid operated valves can as well. Air chambers are unlikely to work long-term. A hammer arrester should. Even they, though, will eventually wear out.