Thanks for your response Kevmond. The way it was explained to me is that hydraulic tappets do away with the need to adjust valve clearances by keeping a certain level of oil pressure in the piston spaces between the cam followers off the camshaft and the valve actuators and, if the oil pressure is low when the engine is idling then these will rattle and then, when the engine speed rises, the oil pressure rises and so gets rid of the gaps. A search on Google seems to confirm this. If it was my car I'd try some Liqui Moly aka Molyslip but a bit of a risk to buy a car with that noise and hope it can be cured