I have just replaced the head gasket on a mate's 300tdi. Head was filled and skimmed, valve seats were cut, valves lapped. I adjusted the tappets, checked them twice, all was ok. We fired it up and it ran beautifully for a couple of minutes. Then the engine developed this rattle. Sounds like loose tappets to me. I'm at my wits end. I have never done a head gasket before but I have owned a 300tdi for 12 years and always serviced it myself. I can adjust tappets blindfolded. It seems as though it's losing the tappets' setting. I'm thinking that the rocker shaft may have worked its way loose. Any pointers?