Something to consider with the "soft" sound shield, at least on my 390, it retained moisture/humidity, leading to rust. Also made routine checks and maintenance unnecessarily difficult. I permanently removed mine for those reasons....rather have a bit more noise. YMMV.
Good observation. Did yours come with the electric exhaust fan? I think that is keeping the humidity in check so I am not observing more corrosion than what you would expect for the age, the use and the environment around the generator.