It's poping in 4 wheel drive because at some point someone has run it on dry surfaces in 4 wheel drive or several times. It is coming from the bevel gear case in the rear . The front end and rear end on a mule are not in sync so that is why it tells you in the manual not to run in four wheel drive on dry surfaces. That is the nature of the mule however you don't want to do that on a vehicle of any kind.