Seriously though, it could be one of several things. A lot depends on the mileage, servicing and use of the little beast.
The auto-adjuster could be malfunctioning or broken. If the oil hasn't been changed frequently it could be gunged up. If it's been to the moon and back, the chain may have worn sufficiently to be slack, the tensioner having reached its limit, or the chain could be shagged in itself.
If there's a rubber coating on the tensioner, this may have worn out. If it's been regularly thrashed from cold, it might all be shagged. It might be a broken valve spring - or come to that, a tired or snapped spring or bit of loose metal anywhwere.
On the other hand, it might be your keys jingling against a bit of metal.
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