Help! XT500 clattering

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Just before Christmas, whilst I was riding my 1983 XT500 around town, a sudden clattering started inside the engine lower down. The bike continued to operate as normal, and the noise continued. Same the next day - clattering sound, like a loose thing inside, but rides normally.

Question: Cam chain loose/broken? If so would the bike continue to ride? I assume not?

Or...

Big-end bearing?  :-\

Or something else?  atch:

 

[Gerard]

 

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[Gerard]

 

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