No need to thank me, I just copied and pasted a link, thank BrianG, the maker of that site!
I could be wrong, but that could just be the sensorless motor trying to find the right coil.
It is called cogging, it looks somewhat like this:
https://www.youtube.com/watch?v=Y7MEWyW5yF0
He does run very little throttle in the video, so it is worse then when you accelerate faster.
It does get worse with heavy gearing if I am correct, so that might be something to?
If you run a lot on low speeds, or want a smoother power band, you might want to consider the new Sensored Castle Combo's?
I am not that knowledged in RC though, I am trying my best, so you might want to have this second checked :)