When does it occur? What valve did you install? Water hammer is the result of water turning on and off quickly. Since the water isn't compressible, when you have it moving, it has inertia (just like your car), and it doesn't like to stop (or go) quickly). Some valves say to include a water hammer arrester in the pipes when you install it. Many designs don't need one. If the pipes were not properly supported during the install, the whole valve with the attached pipes may be banging around. If your water pressure is excessive (anything 80psi or above is required to be reduced to a max of 80psi) that an make the problem worse.
You may need to open the wall up (hopefully, from behind so it's usually easier than dealing with tile) and install a pair of hammer arresters (not an air chamber - an engineered hammer arrester) and/or anchor the pipes and valve properly. If your water pressure is quite high, it may soften or go away if you install a pressure reduction valve.