Paul,
My HJC helmet visor had these snap in vent trim pieces that would rattle and transmit the vibes to the GoPro housing. You could hardly tell shaking the helmet but it was loud on the audio. Glued those, helped about 75%. remaining noise was from the rear of the visor where it comes close to the helmet. It was very close and was flexing/tapping on the helmet. I put some foam tape under this area, problem solved. Basically anything that can move/rattle/squeak on the visor will transmit the vibration to the housing and seem much louder than it is.
I also made a new mount. I used a short GoPro arm with a 90 deg twist(came with camera). Drilled one hole in visor to mount arm. Housing mounts to other side with M5 bolt. More compact, don't even know its there. Maybe a few mm lower than yours(the thickness of the arm) but I have nice vertical adjustment. All is good now.
Some of the straights and sand roads in that run were in the 60 - 70 MPH range on my ICO and you could hear the wind noise, don't know what to do about that.
Most stuff is far slower so not a real issue.