Engine Rattle

[Learjet]

I have an engine rattle on my newly acquired Appia 2. Apart from the rattle it runs smoothly and appears to be quite strong. The rattle is definitely coming from the front end (not the water pump as I ran it with the belt off) so I suspect the timing chain.
Can someone explain how the timing chain tensioner (stretcher in Lancia speak) works exactly. I know there is a cam operated by oil pressure but that is as much as I can discover.
More precisely, if the stretcher mechanism is faulty, I need advice as to what the likely problem is and the best way of fixing likely faults. The parts manual and the repair manual only show as far as the cam and the dog. There must be more to it than that. Also checking with the usual suppliers I cannot find any spare parts apart from the actual chain. It maybe that the chain has simply stretched, but I doubt that.
Thanking you Appia experts in advance.
Simon