ORIGINAL: oldtyme
... I have put on a heavier duty (Dubro) servo arm to prevent any distortion because the standard Futaba servo arms wanted to twist when operating the throttle. The rods are supported in several places so there is no flex that I can determine. The spring is on the carb throttle arm as it arrived. So why is the servo buzzing at full open throttle? Yes I tried another servo.
If your throttle linkage requires a heavy duty servo arm and the servo is buzzing I would suspect that there is a significant amount of drag in your throttle linkage system.
Some servos briefly emit a nearly inaudible buzz as they stop at a null point or they may continue to buzz slightly if they feel a mild resistance from the throttle return spring. I don't know if a throttle spring can be powerful enough to bend a servo arm but you can disconnect the spring (don't remove it from the shaft) to find out if it is causing the buzz.