It may be worth checking out how the cable is adjusted, but if it doesn't do it in the other gears I would see if it has to do with your speed first. Also make sure your clutch is full disengaged (pedal to the floor) before attempting to shift into gear. Mine has a heck of a time going into gear half of the time anyways, but it doesn't grind. If it doesn't turn out to be your speed reverse may be going out, but I don't believe that to be the case. If I remember correctly reverse isn't synchroed, someone with more transmission knowledge may be able to assist further.