So we were listening to the noise while you had the rear wheel off the ground. It makes the same noise when both the rear tires are on the ground and you're driving he machine at very low speeds? Correct?
Do one test for us. A bit of work, but excellent troubleshooting....
Get both rear wheels off the ground, remove the rear tires, remove the rear brake calipers, now put the the rear tires back on and see if the noise is still there.
Rear brakes can tend to create noise due to the wear on the pads. The pads can tend to wear at one end, and that can create "clicking" at the rear. I believe this is because the rears are always pointed in the same direction. The front brakes wear differently as the wheels are always being turned in different directions and that sort of evens out the wear on the pads. For RZR 800 machines, adding springs to the rear calipers, as some of the Ranger models have or had, eliminates the problem
The other thing I can think of is the slop in the splines at each end of the rear axles. The rears have a cir-clip where the rear axle plugs in. Is it still there?
Just a few thoughts,
I had a similar noise that started a month ago and continued to get louder. I was sure it was either a hub bearing or rear crack case. I would jack it up and run the cart in low and high, nothing... No noise. Had my wife drive whilei hung myself of the back lessening to pin point. Lind story short, at least for me it was an easy fix. The bottom most bolt that holds the clutch cover on was loose. This noise has been driving us crazy for over a month now.
Just a thought, hopefully it helps someone save some time and money