I think you need to establish which gearbox you have. At the age of your car I would expect an R380 rather than an LT77, but with MMC you never know........
Easy to identify , is reverse gear underneath 5th ? If so you have an R380, if it is alongside 1st then you have an LT77.
Once you have this data then you can hone in on the correct procedures and information.
I am afraid that Gary may be correct , the troubles solved by the cup and the rubber/PU bushes are difficulty in getting gears rather than a buzz/rattle. I do hope the easy fixes cure it for you. If not then you will need to remove the box from the car and either dismantle it , or get it done by a specialist (of which there are many)
