Water hammer occurs when you close a valve quickly. The sudden stop of the flow of water creates a shock wave that travels back up the line. Placing a water hammer arrestor at the first change of direction in the pipe (working backwards from the valve) absorbs the shock wave. Water hammer is sudden and is not sustained for a long time.
What your describing sounds like the CPVC pipe rubbing against the building structure as it expands due to thermal expansion caused by the hot water heating up the pipe material. The sound starts off gradually and will increase and intensity as the pipe gets hotter, it may even continue after shutting off the supply until the pipe has fully expanded due to the temperature change. You may not hear the sound when the pipe shrinks back as the water cools in the pipe because it will take much longer for it to cool off and thus it will contract slowly.
If its thermal expansion the best fix is to eliminate the friction spots, however this is most likely not an option. Another fix, and one that would eliminate the time it takes to get hot water, would be to install a hot water recirculation system. This would keep water in the line hot, preventing the contraction and expansion. There are some systems that can be installed using the cold line without requiring a new hot water return line. Here is an example of one such system.