Your rig is getting old? Do not panic, lots of trannys do that. Work the clutch a few times, run it through the other gears, slip it into reverse. Sometimes a tranny will decide it likes to go into 1st before it goes into reverse, sometimes it decides it likes 4th gear before reverse. Find out what it likes and do that that from now on.
Mine does the same thing, in fact every manual tranny vehicle I ever owned made a grinding noise when going into reverse at least SOME of the time. Doesn't necessarily mean something is wrong with it that it needs replaced IMO.
Worst case scenario, your clutch is starting to go bad. But the noise doesn't necessarily mean its that just yet.
Most manual transmissions make noise when youre trying to put in reverse and the vehicle is moving forward, even if its barely noticeable.
Also, it is always easier for reverse if you put in second gear immediately before reverse. Just put in second gear and select reverse (without releasing the clutch) and the gear will go in without any fuss...