I've recently noticed a single clunk sound that happens just as I start to reverse. I jacked up the front of the car and noticed that there is a little play when I try to rotate the wheels forwards and backwards. When I was doing it I was hearing a soft clunk coming from the transmission at the limit of the play. Is that normal or am I about to have something break on me? I was initially worried it might be the diff but I have no idea how to tell if the diff is giving trouble.