Driving down the street today and truck started knocking real loud and blowing alot of whitish gray smoke.. so since i wasnt to far from home, i drove it back approximately 1 block. Had our diesel mechanic come over and see what is going on, he shut down each injector one by one, then shut down #4 injector and knock went away, idle was fine. Is it the colprit or not, should i look further?

anybody got anything to contribute?