Shower valve causing hammering
&quot;Water hammer&quot; (the noise) is caused by suddenly stopping the flow of water in a pipe and creating a &quot;shock wave&quot; because water cannot compress and yet has kinetic energy when moving. The cure for water hammer is to have a valve that closes slowly so the kinetic energy dissipates slowly, or to trap air in the piping system so that the air can absorb the energy by compressing. That's why you will often see little &quot;pipes to nowhere&quot; inside of the walls.

There are also specific devices called &quot;water hammer arresters&quot; that are similar and have a little bladder or piston inside that serves the same function.

So the fact that it USED to work fine, points to something having changed. Fixing or replacing just the valve may NOT fix the problem. So for example if you have a piston type arrester on the hot side and the piston has failed, it no longer absorbs the shock wave. A good plumber would know this. So he may know this and by turning the water off to that valve, of course the problem goes away. He may be trying to up-sell you on replacing the entire shower valve because then he tears open the wall to replace it, he will also replace the arrester and this solves the problem.