Is it normal for a differential to make a grinding sound when all wheels are off the ground and running in high gear. the right wheel is turning and the left wheel seem to be trying to turn.
Actually, many of them do it because of the loose clearances within the differential syder gears to axle gears, from ring gear to pinion backlash, ...and in the Universal Joint.
Spider gears are not precision, and normally only turn when the car is making a turn, and then turn just a little. Don't worry about noise under your conditions.
I think I'd be concerned about a grinding noise, but , how much noise is it making and just where is the noise coming from?? What happens if you pull the right brake arm and stop that wheel from turning ??