Gearbox noise

It would seem my gearbox is starting to give up, not a huge problem as I have a spare but I am curious as to what is actually going wrong, I can hear a reasonably loud chattering noise coming from inside the end case, initially I thought it was clutch but it is definitely the box.

What's likely to have started to go, a bearing?

If you can mate don't drive it means when you remove it to fit your spare this can be rebuilt with minimal parts so you have a spare again rather than drive it untill it fully fails. Good boxes are getting hard to find. Sounds like bearing fail on the shaft Or diff mind I am no gearbox expert so could be a million miles away.

I would agree with Johnny, sounds like a bearing on the diff.
Does it get worse when going round a bend to either left or right?

Can't hear it whilst inside the car so couldn't say, it's definitely more of a chatter/clatter rather than a grumbling but I wouldn't know any better.

Either way it will be needing changed.