Yeah that's it. The chain tensioners use oil pressure to keep the chain tight between the two camshafts on each head. As they wear inside (I think its o-rings) they develop a tendancy to leak away residual oil pressure while the engine isn't running and they lose their push on the chain so it gets loose. When the engine starts again, the inlet cam is pulled so far by the driven exhaust cam, but then when it gets over-centre on a lobe it jumps forward, pushed by the valve springs. It then pulls the chain tight and slaps it on the tensioner pad - that's the noise that makes the rattle. After a few rotations of the engine there is enough oil pressure to tighten the chain and it stops making the noise.
It starts with rattle on cold start, but Jim's had got to the point where it would rattle on hot-start too so the tensioners are losing pressure quite quickly so need changing asap since the slap of the chain obviously doesn't help the life of the plastic pads.
I've tried changing the oil retention valves which keep oil in the heads on shutdown but they have no effect. Its oil pressure leaking past the piston in the tensioner that is the problem.
I did have an idea about using a thing called an 'Accusump' which is basically an oil pressure accumulator used on race cars. This would collect oil under pressure while the engine is running, store it while it's off, and then release it all when the key is turned. Theoretically this would put pressure in to the tensioners before the engine fires and therefore stop the slap. This would be cheaper than replacing the tensioners (£576 each!!), but wouldn't actually fix the problem of the tensioners bleeding off oil pressure. I think it might actually mask it such that you wouldn't know there was an issue until they bleed so much pressure that the engine has an oil pressure problem.
I did see someone post on FB about rebuilding the tensioners with new o-rings which is worth investigating. I can't see how they come apart in a fashion where they can be put back together but I have some dead ones which I can cut in half. At some point we're going to run out of new ones so we need to find a solution of some sort!
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