Low foghorn noise after new pressure reduction valve and expansion tank installed
Instead of flowing air which is used in instruments like flutes or trompets,  oscillations can be produced by flowing water as well. The complete system consisting of the spring in the reduction valve, the tube dimensions and the expansion tank could be in resonance depending on a certain pressure and water flow.
It should be influenced by the pressure that can be adjusted turning the black part of the reduction valve.
If there is a water heater downstream of the pressure reduction valve, a safety pressure relief valve must be installed, since the expansion tank could fail and the reduction valve would close if the water is heated up thus preventing the pressure to be relieved into the upstream net.