hi, its hard to work out whether something has broken, worn or come loose from your postings. one end of the mainshaft is the clutch nut and the other is behind the gearbox outer cover where the clutch pushrod goes through and should be held with a tab washer. the a10 gearboxes are pretty robust but its easy to take the front cover off and tap with a copper mallet and have a look for endfloat on the shafts. another suspect is bearing failure but to go further you would have to take the clutch of first.
Thanks for info. Just had another look. Run her up and once warm, from stationery, selected 1st/2nd and back to neutral, all seemed fine but tried same sequence again and while in neutral the noise started. ( it does this while stationary while in 1st with clutch engaged occasionally) if you operate the clutch a couple times it goes away. It's a kind of really loud ratcheting sound, almost like if you pull the kick starter down whilst it's running, but louder. I took off the primary case and only thing I noticed was the metal plate that sits around the clutch was warped in places (if that makes sense)
Any advice appreciated before I'm going to have to take to garage mechanic.
Cheers
Paddy.
Oh also if it's starts this noise, if revved up it goes away.