I think the cam chain is so loose its hitting its tunnel. Assuming the adjustment can be taken up as described in the manual, I would still be concerned about the state of the tensioner. Can the lower tensioner be replaced without removing the head? The upper tensioner is available at David Silver, but not the lower one?
The lower tensioner can not remove with the head on. The stud are too long, the bottom of the tensioner fouls on the crank. The guide is also held in place by the cylinder head.
If you go ahead with replacing the top tensioner, take great care not to drop any bolts!!