The preamble : about 15 months ago I treated myself to a nice modern helmet - carbon fibre, internal visor , nice graphics etc to replace my $ 60 special. I was disappointed to find it was so loud, and there was a thrumming kind of buffeting above legal speeds that 100 kph here). I have heard that the thinner carbon fibre is more prone to this.
It did annoy me and I bought an MRA Xtream add on to the standard Mark IV screen. It helped a bit but still wasn't the best - doesn't look great either. I just persevered.
Then, I had the misfortune to break my screen. Had it replaced by Eagle Screens in Perth ( same profile, greyish rather than brownish tint). To my pleasant surprise, the buffeting has largely gone, even at high speed. If I tuck in lower, the buffeting increased, but in a normal position, it's right in a sweet spot. Didn't even bother trying to re-fit the MRA extension.
The original LeMans screen has a plastic edging on it ( I think this was to make it comply with Euro regulations?). The new screen does not, so obviously this was creating turbulence right where I didn't need it. I am a very happy camper !
So, if you have a screen with that plastic beading on the edge and experience buffeting, maybe try removing it to see if it improves.