Thanks for the tips about the starter. I haven't been able to figure out exactly where the sound is coming from, but it does seem to be in the same area as the starter.
However, I found this article which describes a similar rattling noise in SOHC KA's:
http://www.240sx.org/faq/articles/startup_rattle.htm
Does this same problem occur in the DOHC engines? I just had my timing chain tensioner replaced and I happened to stop hearing the rattling for a couple days after that. But I'm not sure if it was just a coincidence since it was during really hot weather and I typically don't hear the rattling when it's warm outside. Also, the day the sound came back was the same day the weather started cooling down.
Any other ideas? Thanks.