Hi. Am I need of help again. I have an 88 nc23 , finally got her through the mot a couple of weeks ago and done around 200 miles since ( no problem. Went to go for a ride on Sunday and within 20 yards I noticed a intermittent knocking noise like something was loose or catching. It sounded like it was coming from below the seat. I’ve lifted the rear wheel and spun by hand, no noise / rattle. When run in gear the rattle/ noise appears. Is this a gearbox issue?

probably your chain/sprockets is too loose/or in poor condition.

Does it go when you pull the clutch lever?
Could be the clutch basket or perhaps a wheel or sprocket carrier bearing?