IMO< I would suggest Draining the rear diff, and look at the color and amount of oil that is in there,
to re fill, fill till oil comes out the top of the fill hole!
drain into a clear container and look for water or metal in it, can place a magnet on the bottom to pool any metal to one area too!
could be a bearing on diff that is grinding and is DRY and as heat builds, it starts to seize up and makes your grinding noise!
I would also suggest that you ride atv till you hear noise gain, then jack atv up , all 4 tires off the ground and try and pin point the sound better then!