Oil filter. Mine was doing this for a long time. I got a premium oil filter (Mopar) and it no longer does it, well, have a serious knock now due to valve spring failure ('03) But the cheaper filters let the oil bleed back down from the valve train due to cheaper and weaker anti-flowback valves and springs in them. Change the oil and upgrade the filter, don't really want to go cheap here anyways, after all, it's the life of the motor right? Hopefully this helps some of you guy's with cold start up clatter, it cured mine. There is probably a sticky about the filter somewhere here, not sure. If not probably should be.