Top end rattle after head gasket replacement

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?

Sorted. One of the pushrods wasn't fully located on the cam follower so when it finally located itself, the gap between the rocker arm and the valve grew substantially.

You're lucky it didn't bend a push rod or break a rocker arm or shaft.