On the early 560s, (117.968) the cams were made of a softer alloy and the lobes tended to wear prematurely. I think this was remedied in the late '80s.
Anyhow... a constant clacking can be indicitive of cam wear or rocker arms problems. You'll have to inspect the cams to get a better idea, or perhaps you can use a piece of hose (like a stethoscope) and try to islolate any noise -- that is if there is any(?)
I'm dealing with this problem right now and hoping it's the rockers, but my brake job takes precedent at this point.