Re: Random scratching noise inside clutch shaft
My master link came off and the chain got caught up in the 10 tooth sprocket a while ago.
It must have put some stress on the clutch bearing on the sprocket side, because now it makes the same sort of scratching sound you describe. Some of the balls must have become deformed in the race, maybe.
I have ridden more that a 1000 miles since that happened and no problems so far.
I used a chain breaker tool and now I run the chain with no master links, so that problem won't happen again.
The point is, if it is a minor, occasional scratch you might not have to worry about it too much.
If it really concerns you, just pick up a new engine lower half , or even a whole engine, and swap it out