Gearbox noises in 80 series

For some reason my 80 series gearbox is making extra noise. This has come on rather suddenly and is most noticeable in 4th and 5th gears. It's only done 300,000. A slightlty, rattly metallic sort of sound that I guess is bearing related. The gearbox continues to work perfectly (including the hard shifting when cold from 1st to 2nd and back). I'm looking for suggestions as to what to do given I'm a poverty struck Melbournite with a view to a big trip in a year or so. Should I try a different oil or bite the bullet and fully reco gearbox (?cost) or any suggestions re second hand? Anyone know anyone knowledgeable about L/C gearboxes in Melbourne? By the way its a 1HZ motor. Many thanks in anticipation