well, the obvious first step is to open the hood, put the car in neutral
with the parking brake on, and start it, then listen and look - is the
chatter from the front, rear, side, or what? is there something near where
the chatter is coming from that could be doing it?  and so on.  On e
possibel culprit is valve lifters, but they dont' sound like a plastic fan
to me.  another possibility is the fans themselves - but you need to look,
or pay someone to look for you -