Take some Velcro (soft side) and cut in 1" lengths Then fold each in half and cut a small V notch in center (a punch will not work) and peel the back off and place one on top of each mounting studs.
The sticky side keeps them in place if you ever need to remove the housing in the future. If you use rubber O rings, they tend to fall off and flatten out.
I too have a vibratinal buzz at certain rpm's. I can't tell where the heck it's coming from. Doesn't seem to be the speedo, I've pressed down on it in various spots and it doesn't stop. Headlight? Maybe. Should a person just go ahead and do the anti-vibration fixes or is there a sure way to locate this noise?