Probably cam chain but could also be a primary chain rattle. IIRC the cam chain tensioner release bolt is on the back of the cylinders, in the middle, just release and retighten the nut.
A primary chain is a different matter, I seem to recall that Honda fitted a tensioner to the 650 which the 500 and 550 versions of the same type engine didn't get, if so your lucky as it could just be stuck. However it is an engine out job to check this so try the cam chain first.
Oddjob
September 2010
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