[accuab]

More than likely it is the cv joint that is popping. When you are rolling backwards and shift into drive, the play in the joint comes out of it and pops when it connects. When a cv joint gets old, it forms play in it. That allows the transmission to turn and not lock in immediately like it should, so it pops when it locks into the cv axle. I would have it checked however to make sure that you don't waste any money on fixing the wrong part.