i've had the little catch that prevents accidental engaging of reverse gear jam before, that prevented me getting reverse gear but didnt involve any grinding noises. its an easy check though; if you look at the slots where the gear stick selects the gear (you'll need the transmission tunnel out) you should be able to see the spring held flap, mine was only held on with one screw and had gone squint preventing it from lifting. if your lucky you may hae a similar easily resolved fault!