Sound from rear
Two weeks ago I noticed a buzzing sound coming from the rear of my car and I have been trying to diagnose it. It only changes with speed. Starts at around 25 mph and gets a little louder and lightly higher pitch at higher speeds, but not a lout roar. Other than changing speed, there is no change in the sound when accelerating, braking, turning, or shifting into neutral. I put it up on a lift and noticed the rear wheels (both) have a little more resistance when I turn them. I give it a shove and it rotates about half what the fronts do. They also give a bit more noise too. This occurs with the brake pads and caliper removed as well. I also turned on the vehicle and put it in drive and noticed that while with no gas three of the wheel would move at about 30 rpms, the rear drivers side when would barely move. I used a stethoscope to listen and I could hear nothing from the rear differential when manually moving the wheels nor with the engine on and gas pushed in. I could hear noise when I put the stethoscope against the metal shield behind the rotors. I am not sure it is the same noise causing the buzz i hear.
Any thoughts on what this could be and any further tests I could do. The car has 78,000 miles on it and is a 2013.