I'm getting some noise from the rear discs when I shift from D to R. I guess my thoughts on this were that when the driveshaft begins to turn the other way, the rear brakes don't really hold it causing a little sound. The vehicle doesn't move because the front brakes are holding it still.

If I push the brake pedal further in, it doesn't make the sound any longer. I assume the noise ceases because the added pressure from the rear calipers keeps the rear tires from turning at all.

Has anyone else experienced this, is it normal, or should I take it in?

Completely normal