2003 Base 2.4L rattle at cold startup

My 2003 2.4L has a very noticeable rattle or clatter coming from the front or top of the engine a few seconds after a cold startup. It lasts for maybe 30 seconds or a minute. Then the radiator fan seems to kick into a higher speed (??) and the rattle stops. Has anyone encountered or heard of this problem? Does the fan in fact have 2 or more speeds or is it my imagination? If so, is there some kind of relay involved that could be causing the noise?

 

I checked this again yesterday. This time the rattle stopped before the fan kicked into the higher speed. So they may not be related after all. I also realized that it's not a question of a multi-speed fan, but the engine kicking into high idle, and of course the high idle would make the fan spin faster. Duh! (if this truck had a tachometer I would have known that right away). Anyhow, still trying to figure out what the rattle is.

 

On mine the ac compressor clutch makes similar noise on cold starts for about the first 30 seconds......

 

I've heard about that, but my AC hasn't been working in years. Right now I'm leaning toward piston slap.