I started my truck this morning and it has always been kept up with, regular oil changes and maitenance. Well, I started it and it was tapping pretty loud until about 10 sec later it want away. The oil pressure was fine, the oil level was ok but what in the hell would cause this all of a sudden? It does not do it when you start it up warm but only after sitting up cold for about 3 hrs then it will do it. I am thinking a lifter or loose rocker arm??? I want to keep this truck but I am not sure where to start. I know it nothing major like piston slap or a rod because it's not missing or running weird at all, its just knocking at cold start. Can anyone help?