Rattle Valve Train Noise at Start-Up

3.0 DOHC Duratek Engine on a 2002 Sable with 125K on it: This loud rattle/tapping noise just started on our way home from Nevada last Saturday (9-7-14). It only makes the noise at a cold start-up for about 10-15 seconds (or until the oil begins to lubricate the valve train), then the engine is quiet. After the engine is warm, it doesn't make this noise at start-up anymore until the next "cold" start. This car has always had regular oil changes with full synthetic oil and I changed the oil just before we made this trip to Nevada (Kansas to Nevada and back again). Does anyone know what is causing this rattle/tapping noise? Any suggestions would be greatly appreciated. Thanks in advance.:eek7:

This noise completely went away and I never did discover what caused it. The engine runs great now and is very quiet, even from cold starts. Has this happened to anyone else?:uhoh: