High pitched whistle from engine at speed

I've had my boat (2009 MB with PCM 343 engine) about 4 years and last year really started to notice a high pitched whistling sound from the engine at about 20+ MPH. It's not extremely loud, but it's moderately annoying.

My (independent) mechanic said it's just the air passing through (something? I'm an idiot with engines and didn't catch exactly what he said), and that it is normal and not something that can be changed.

Just wanted to get other opinions on this. It seems very strange that this is "normal".

My dealer closed a few years ago so I don't have another expert to ask.