The shower in the second floor master bathroom makes a scraping noise that radiates to the bedroom below and disturbs the occupants. I have tested some theories. If I remove the shower head and run the shower, the noise stays the same. So the problem is not the shower head. It is not a water hammer. The installation is NEW, so it is not corrosion or build-up on the parts. I installed a new cartridge and this had no effect.
The unit is a single mixer with a diverter to shower or hand wand. The noise occurs no matter where the temperature mixer is set. The only thing that reduces the noise is installing a flow restrictor in the shower head. But the flow is then too slow, hardly a normal flow rate. This solution is not acceptable. The noise seems to be related to the volume of flow. I don't know how the pipes are connected, they are inside the wall. Could there be too many bends in the pipes, or a narrowing of the pipes at some point? I SHOULD(!) be able to get a full flow withoit the scraping noise! Someone suggested it might be similar to the noise from a hose bib, but how do you fix that?