I hear the rear clicking noise in my 570 when going into reverse as well. I also having trouble finding reverse more so than other gears, not sure why. I will be in reverse but you can tell the gear hasn't caught, and I have to tap the shifter to get it to find the gear and go.
I just attribute it to a characteristic of Polaris. I've always heard their UTV's have transmission issues so maybe the noise is an indicator of trouble to come, not sure, but hopefully not.
Anyway, I will follow up to this post if I ever run into any transmission problems that can be linked to that clicking noise.