2014 570 RZR LE Stock with Versa Track Turf Mode and Engine Braking system, around 400 miles on the odometer.

Description of problem:
After riding for a few hours on road/trail with little or no hills at 5-40 MPH with transmission in 2 wheel drive. Trail becomes steep and switch to On-Demand 4 wheel drive and low gear at times. After descending steep down grades and Engine Braking System engaging numerous times, begin hearing what sounds like “thumping” nose from rear on level ground at slow speeds. Increase speed and it appears to stop or go away, but if I slow down it starts again. If I run at 20-40 MPH for a period of time and slow down problem seems to have gone away.

Anybody got any ideas what the problem is or is it going to require a visit to the shop?