I tend to agree with bugman that you probably have a vibration problem which occurs at that particular RPM. I also agree that using a non metalic push rod on the throttle is good insurance. I often violate the 12 inch rule and have never had a problem with my battery and electonics much closer to the engine compartment as close as 5 or 6 inches. I'm running mostly Quadras with magnetoes and no shielded plug wire but do use resistor plugs and am very careful to never have any loose metal to metal fittings anywhere on the plane.