My house is piped in CPVC.I changed the PRV Thinking that would do it.That did not work.Every time you open any tap in the house I get a high pitch noise though out the house. It sounded like it was coming from the PRV. Note there is a 50 gal. electric water heater right next almost in front of to the PRV

Noise and squeals are caused by vibration. Can you feel anything around that vibrates?
If the PRV didn't catch it the next thing I would check is the washer in the house shut off valve. If the screw loosened then that could be the source. Is there anything else on the cold water feed before it diverts to the water heater like a filter, watesoftener , etc? Let me know, Tom