The Hydraulic tappets work via oil pressure to sustain them, usually a oil seal around the top of the tappet goes thus allowing the oil to leak out. This means that the hydraulic tappet requires replacing. If you are lucky only one, maybe two will have gone. Last time i bought one of these for a Ford they cost between £10 and £15 each. The only way to check for sure is to remove the rocker covers and inspect the hydraulic tappets, and knackered ones will be loose on the top.
i thankyou