ORIGINAL: acerc
Odd's are your past the end point of the throttle butterfly just a little bit. Just back the endpoint up till there's no buzzing. Once you fire it up you will find the wide open, by rpm's, is far short of wide open at the butterfly.
+1
Your described approach to setting up the throttle servo and linkage is perfect for a glow engine for which the response is linear. Not the same for gas. Install all the components get her running and make adjustments from there.