:confused: I've developed a noise that can be characterized as the same kind of tone you would hear from a tuning fork when struck. This happens after sink faucets have been turned on and then shut off. I had a hot water re-circulator installed and developed the noise shortly afterwards. I called the plumber who did the install and he replaced the pressure regulator to no avail. He returned to swap it out for a new one since they occasionally can be defective. No help with the harmonic noise which can be heard all over the house. So, he installed an expansion tank at the water heater. All of these have provided short term fixes but this loud harmonic tone always returns. I even drained out the system but still no solution. Now the plumber appears to have re-located and I still have the problem.