Fluttering/rattle sound when accelerating Prius C

Hi all, I have a 2014 Prius C with 107k mi, that, since at least 75k, has been making this odd plasticky sound when accelerating after intial start-up. The sound seems to go away once engine is warmed up. It sounds a lot like that old playing card in a bicycle tire, but lower pitched. I've looked at all the forums with similar issues but can't seem to find the culprit. I dont think it's an aero panel or loose anything as it's definitely tied to acceleration and goes away eventually. Anyone have any idea?