I have noticed this before as well, and believe the noise comes from the engine. It seems to be some kind of harmonic noise which travels through the throttle cable. When you push the lever forward hard, the sound is not able to reverberate in the cable. I have no idea what to do about it. I've noticed this before on Magnum tractors with mechanical engines as well, tried rerouting cable slightly - no change. It's funny how these noises occur but they seem to come from vibration. We had a Massey 850 that had a noise like that in the main drive lever - it didn't shake, bearings in pulley seemed fine, but it would sing at you while you worked. Maybe it occurs after some of the inner lining of the cable wears away, replacing it may help. The cable is like a phone line direct from the engine to the cab.
