quiet when cold, rattles like mad when warm ??

[Dead Eye]

What makes you think that it isn't the cam chain?

Only real way to check is to remove the tensioner and see how many adjustments you have left, but that's not exactly the easiest job, especially if you have the carbs still in place. Plenty of other stories where the cam chain has rattled for 3-4k before it finally adjusts. I wouldn't have thought you'd need to replace it yet though...