Tappety noise after hills

It's a bit of an odd one this. A month or so back I changed the oil and oil filter and put in some Castrol 10w40 and a couple of starts after that the tappets were quite noisy when the engine had just been turned on which lasted for a couple of minutes. Added a bottle of Wynns hydraulic tappet magic which made the noise quieter and not last as long.



If I turn the engine off for longer than a minute the tappet noise will be back, even if the engine is at full operating temperature.



I've also noticed that if I go down a long or steep hill where I'm off the throttle for over 30 seconds then the tappet noise will come back. There will also be a burning smell when coasting in gear down hills(not tried coasting in neutral and don't particularly want to)



Any ideas what this could be? It was fine before I changed the oil and it's driving me mad.

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