The rear brake noise is due to the location of the rear brake. That simple.
The cure for it is to put the rear brake in a different location. The noise is inherent to the design, and not just the Rhino. The Yamaha Grizzly, and the Honda Rincon ATVs have the same issue.
because the rear brake is located between moving parts, the transmission and the rear end, CVs, and axles, there is "lash" between parts on both sides of the brake. When braking, a harmonic is created when the lash is taken up and released, over and over again, on either side of the brake.
The noise is subject to amplification if the rotor and pads are in poor condition. However, the noise is likely to occur at some point.
I used to be an ATV Test Engineer at Honda. I have had many discussions with engineers who specialize in brakes who have educated me on this subject.
Hope the explanation helps.