It's just a first guess Ozziesim, but SB45s with the original clutch that uses a captive cotter to retain the engine side of the clutch, are very prone to running out of true after some wear develops in the cotter and the fit deteriorates between the engine's PTO (output shaft) and the aluminium clutch. When there is slack between the shaft and bore, when the cotter is tightened up it forces the clutch out of true with the PTO, and that makes the clutch run out of balance. The vibration itself is the least of the problems - it causes the engine mounting deck to fracture at the engine mounting bolts, due to metal fatigue. Of course the out of true condition gets worse over time, due to fretting wear between the shaft and clutch.
