There is a bug since launch that no matter in what car with a turbo you sit in, the blow off will do the same sound no matter in what RPM or boost stage you are. I remember that in Forza Horizon 4 that worked like a champ. It’s a bit annoying considering how nice this game is.
I am not the first one who noticed/reported this issue.
Hopefully this is some of thing which could be changed.
Greetings!