My car is making a noise when I think it is the bearings in the gearbox?

Hello,

Is the transmission making a noise/rumble when stationery and engine idling?
If yes, then try and see if the noise appears with the clutch engaged or disengaged?
If noise appears with clutch disengaged, then the most likely issue would be the release bearing as you are applying load on this part when disengaging the clutch.

How to replace a release bearing.

https://www.2carpros.com/articles/throw-out-bearing-replacement

If the noise appears with clutch engaged, ie the transmission input shaft is turning then the most likely issue will be transmission bearings.

A transmission description.

https://www.2carpros.com/articles/how-a-standard-transmission-works

If this is the case, then first check the transmission oil level.
The best way is to open the filler plug, 17mm in green on picture, and see if any oil runs out.
If no oil emerges then add oil until it starts flowing out of the opening.
See 2nd picture below.

If you need to add any oil use MTF-94 specification 75W80 transmission oil.

The location for both the drain and oil filler plugs can be seen in the 1st picture below.

A good description on manual transmission noises.

https://youtu.be/6WfPnb7MUCM

Please let us know how you get along.

Cheers, Boris