The post-1975 CB750 has a sensitivity to being poorly tuned that makes the clutch rattle. So, check out the points, plugs, oil type and weight (it should be 20w50, not 10w40 oil), valve lash settings, cam chain tension, and carb sync. All of these things have a tendency to be a little off after the bike has been sitting a long time, which will make the clutch rattle. Especially true is the tendency to make noises if the points are off and the timing is not dead even between the 1-4 and 2-3 points sets. (In other words, it might need a tune up).
If the clutch dual-steel cushion plate has worn rivets, and if the 'slipper' plate in that clutch is also worn, the clutch will still rattle. This latter symptom usually results in clutch slip, though, especially with heavy throttle at higher speeds. So, the symptoms can be separately addressed, without opening up the engine, to determine what might be causing the noise.
