The installers done nothing wrong, your jumping to conclusions as usual, everyones a numpty of course
They make the noise even on the correct setting. Me and my old tech eng came up with the figure 8.5-8.6% after some experimentation. Perfect everytime, doesnt affect the gas rate and makes the noise go away.
But if I encountered that problem then I would normally expect to make any necessary adjustments myself in association with the maker's tech help.
When I fitted one of the first Isars, I had that problem and although I told them what the problem was they insisted on sending a guy from their design team and the regional technical manager. At least thats what he seemed to describe himself as. But he did not seem to have any technical knowledge.