Is there an expansion tank in the water line near your water heater? If not, that is likely the problem. As an experiment, buy a water pressure gauge (around $10) and screw it into the drain of the WH or somewhere else you can easily attach it. When things are making noises, check the pressure. If you have a PRV and don't have an expansion tank, after running the hot water, the water expands, creating a potentialy very high pressure. This can make the prv make noise sometimes. The PRV has an internal check valve causing the system to be referred to as a closed system. A closed system needs room for the water to expand as it gets heated.