Knocking/Tapping coming from PRV

[Jeff H Young]

turning off appliances for a day shouldnt be a big deal , if noise appears , turn back on thats not problem. water heaters can rumble , pop, make all kinds of noise. only thing we can do is throw every idea we can think of out there. Ive yet to hear the noise. could be some kind of debris running through pipes that flushing out might fix or create a change in noise , move it to another area. but we basicaly know you changed regulator and it started when? next day , same day, week later? What about the work that was done changing meters when was that done? the process of draning water down forces water in opposite direction jarring things loose that may have lodged and not made noise previously and now just kind of bounce around.
you could try draining water, shop vaccing it blowing compressed air back through an angle stop or rear hose bib on house. many double union prv.s have available a jumper which installs easily and is just a straight pipe 75 or 80 psi should be safe try that for a day. the things you can try are endless, takes either be proactive or put up with it. We get this with a lot of issues the OP writes a few sentances and we gotta ask 100 questions . did you try turning off the hot water valve? you said running water might make differance ? have you tried hot and cold? never did find out if house has expansion tank? there are a million things to try.