I own (2) 500 CF Moto Z forces and they both hum when engaged in gear and going slow. Reverse or forward but it is much louder in reverse. Do they all do this? Is this normal?

I was driving and let off the gas and the UTV started making a loud thwaping sound. I thought the drive shaft had broke and was hitting the undercarriage it sounded just like that. I stopped to look and didnt see anything immediately. So I turned around to drive back home and the noise would happen only when I let off the gas and the unit would naturally decel. If I applied a steady pressure on the gas to keep a load it was fine (I would push the gas and brake at the same time to slow down to stop). When I got it home I noticed oil leaking out of the rear end. I assumed that was the problem.

I currently am taking the rear end out but I am not sure if that is the problem or result of another problem. Does this sound like what would happen if the rear end was bad? The rear differential turns fine.