I had a similar experience running my BMW shaft drive bike with the rear off the ground. I believe the noise is caused by backlash in the transmission, it only becomes evident when it is running with no load, and is normal. If I recall correctly, it is not advisable to run the transmission without load like this.
Regarding the wheels rotating at differing speeds, this will be due to the differential and I think is also normal. No doubt someone with more technical knowledge than me will be able to elaborate.
Colin
