well, they just called me and said there is nothing wrong with the rattle. i guess it's normal then?

I've also noticed the same type of rattling on the loaner G35 with only 15k mileage they borrowed to me today, as well as the other two 08 Maximas (one with 26k mileage, the other has 16k) I test drove two days ago.

 

Have you tried replacing/adjusting the belts? They tend to make noise that sounds like metallic rattling when they get older.