A friend of mine had a similar problem,on a small block chevy,I asked him some of the standard questions .Did you use break in oil or an additive,did you preload the lifters and he said ,what?.And then I knew that he had hyper extended the lifters and he still ran the engine so he proceeded to actually break the keepers on 6 of them and the pieces went everywhere and blew the engine.You didn't sound like that type of person but thought I'd chime in anyway. Shadyj