Do you have an expansion tank? With a PRV, you need one. The PRV could be getting old, but when you have one, you must have a working expansion tank. When you heat water, it expands. The PRV won't let water push back into the main (normally). The ticking could be from expansion from the WH, but regardless, if you have an expansion tank, it could be bad and need replacement, or if you don't, you need to install one.