I have checked it on my scooter. Since recently, I have a similar vibration and sound at about 55 km / h or more. Scooter goes well. When I ride uphill at high throttle, but less than 55 km / h do not hear anything. If I drive down the hill with no throttle (speeds over 55 km/h), also can not hear anything. Just heard on a straight road at a speed exceeding 55 km / h. Really strange.
I think the problem is not in the machine because it works well. Good to hear. Perhaps something in the transmission? The point is to be heard at a speed exceeding 55 km / h and a straight road. I do not know what to go on watch. I feel a strange vibration and sound similar to the description of the previous posts.
I have 5500 km on a scooter. 3.5 years.