Don't know if this is relevant but here it goes anyway.
I have same kind "issue" when I installed mmm combo to my truck. Before that I had feigao motor in it and the gear sound was smooth. Im quite sure that the "issue" in my case is that the cc neu motor has stronger magnets and its harder to turn the shaft. So it's doing the sound for the gears when pushing the truck. When running the sound is smooth.
I know im comparing apples to oranges, but could it be samekind of issue?
Edit:.Typos & some clarifyings
