Well,
if you haven't been tinkering then that answer that one.
Have you been doing the "pedal up" procedure?
What manuals do you have to look at? I am suspicious as to what you have judging by your questions.
What is your skill level and shop equipment available.
I am in the process of acquiring some gearboxes as spares for rebuilding, I have had mine rebuilt locally but that is all long term, you want here and now fix.
The gearbox is relatively simple but it seems light years ago that mine was rebuilt and the vehicle rebuild has stalled owing to building a shop.
There is a lot inside but it is a drum and brake bands and gears inside. The photo shows one apart, mine, a lot can rattle.
Give us some answers, sounds like gear specific problem.