Building a trainer

[Espeefan]

If you are sure the buzz is coming from the motor, then I can only guess that your neutral throttle position is off a little bit. The speed controller is actually giving the motor some juice at neutral. Not enough to actually make the truck move by itself, but enough to make the motor buzz. This is common noise at the very slow speeds, or when starting a model from a dead stop with a really light throttle application. The moments before the motor begins to spin.

If you are sure you didn't bump the throttle trim one way or the other, you might need to recalibrate your speed controller. In other words, re-teach it neutral, forward, and reverse. Check the manual to see how it's done, if you aren't sure.

Otherwise, it's possible your radio's throttle stick is not centering itself in neutral very well. A weak spring might be keeping it off neutral ever so slightly. The way to check if this is the problem is to turn the radio on, turn the truck on, and then barely move the throttle stick forward, or in reverse. If the motor buzzing stops, and the truck doesn't move, then that is the opposite direction the neutral point on the radio or the speed controller is off. Or you could try adjusting the trim one way or the other, on the throttle channel, until the buzzing stops. I wouldn't use trim adjustments as a permanent fix, but for testing it will let you know what is going on. The correct fix would be to center the trim, then calibrate the speed controller.