Mitsubishi’s manual gearboxes from around this time (and earlier) were some of the noisiest around. But usually it wasn’t anything to be concerned about. Bearing rumble and some clutch bearing noise were pretty much par for the course, but your problem sounds more like it’s associated with the gearbox’s synchromesh rings. These rings are designed to allow the gearbox to slip between ratios smoothly and silently, and any crunching noises during shifts can often be traced to worn synchromesh parts. The good news is you won’t harm the transmission by continuing to drive it, particularly if you’re careful and take your time with the shifts you know are most likely to create a crunch. If you can live with that, fine. If not, the gearbox will need to be removed from the car and rebuilt. That will be neither cheap nor simple.