999/1000 times the rattle noise is caused by the tabs on the frictions disks bouncing back and forth between the fingers of the outer basket and plate teeth rattling against the clutch inner.
On clutches with lots of mileage the baskets may have notches made by the disk tabs and plate teeth, which increases the space and hence the rattle. In combination, the tabs and teeth themselves may have been worn, giving similar results. The obvious solution is to replace the worn outer basket and the friction disks, less often worn inner basket and clutch plates. Manufacturing tolerances being what they were, there is a chance that any new parts will be that tiny "thou" smaller and so swapping will not fix the problem.
It may depend on a particular clutch as to which noise is more pronounced inner or outer. Usually outer is the culprit.
Stronger springs tend to hold steady one source of the vibrations that cause the clutch to rattle (i.e. they hold the inner clutch more rigidly). However, the main vibration is caused by vibrations in the primary chain coming through the main transmission shaft into the clutch outer. There is no way to dampen the clutch outer except by balancing the outer basket.
Unfortunately this still leaves mainshaft vibrations as a major source. Hence, 1/1000 times, work is needed on the primary chain and chain damper, to quiet the clutch. (edit: Maybe I should reword this: Any work to quiet the primary chain will quiet the clutch, buts its not usually necessary in all but 1/1000 cases....better
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Finally higher viscosity oil plays a part. The thicker the quieter. Unfortunately, the trade off is in difficulty in shifting as the heavier oil has the effect of closing the tolerances especially of the dogs, but also shims and shift fork.
So, if your are looking for the holy grail of clutch rattle, I hope this has helped.
