Horrible noise at cold start, LE 2001

[Cortlandch]

I cannot identify the source of this horrible loud noise at a cold startup. The engine runs completely normal, and has no issues after starting and warming up, but if the vehicle has been sitting for a few hours or more, it makes a pretty awful grinding or rattling noise for a few seconds. Once it's been started, it won't do it again for several hours.
Here is a video I took of the sound:
http://tinypic.com/r/2wlwqpd/9w

Is there any ideas to what this could be? It sounds almost external, but when I watch inside the engine bay, I can't see a thing that's rattling or out of place. Some suggest it's the fan, but that wouldn't make sense, since that would mean it would happen more frequently, or at every startup, but this only happens once every few hours.

Thanks for any expert advice!

[smj999smj]

It sounds like a timing chain rattle to me. Check your oil level. What kind of oil filter are you using and what's the oil viscosity of the oil that's in there? Has the vehicle had its oil and filter changed at the recommended intervals?