Check your nuts.

[Clive]

Started getting a strange noise and vibration which appeared to be coming from clutch casing area.
Got side case off and was having a general look/check around.
To my horror I found that the primary gear had a small amount of backlash and the oil pump drive gear could be spun on the shaft.
It turned out the castellated nut which holds the gear etc on the crank was loose, not enough to be moved by hand but loose enough to allow the above.
Of course my first thought was big end and main bearings would obviously be starved of oil if pump drive gear was not operating as it should.
Generator side case off, and sure enough I could feel a tiny bit of movement on the flywheel, again not a lot but enough to be noticed.
I was going to just tighten the nut, put it back together and hope for the best but on reflection I felt it was just delaying the inevitable.
So looks like a strip down and at least a new main.
I'm only glad I noticed it straight away and rode the bike straight back home or it could have been a real disaster.

[3potjohn]

I had this some years ago after rebuilding my 350 engine (for the 1st time). I recall the noise as the gear slipped, not all the time but as i changed speed.Sure enough the gear was slipping on the shaft as the nut was not fully tightened. I just tightened it up and ran the bike with no issues until this year when i rebuilt it again. But if you can feel movement at the flywheel then maybe it will need a strip?
John

[George 350]

If you have ANY movement up/down of the crank/flywheel, don't put off the repair, otherwise the cost will rapidly spiral when you realise you need a new stator and at a minimum have the flywheel remagnetised.
On a rebuild a few years ago, i knocked the 'green neoprene' sleeve off the end of the oil pump when fitting the primary case and the main bearing and the bits mentioned above were toast in just 10 miles.
George.