So, I have been experiencing strange behaviour from my Ninebot Mini. When riding at slow speeds there is a very obvious vibrating sound from the motors, kinda like "wrr, wrr, wrr, wrr" and I can feel it vibrate. As soon as i accelerate and ride over a certain speed this vibration and sound disappears and the ride is smooth. This behaviour started about a two weeks after I bought it, but since it didn't really effect the riding I thought it was no big deal.  But just recently I started to fe