Alright, I might be crazy, but I definitely noticed a thumping sound from behind the passenger side wheel well when walking around my car yesterday at idle. It sounded like something knocking into the plastic shroud at about the same speed as the engine, and would cycle on and off. Air conditioning was not on at the time. Is there anything up there that could be knocking around? The sound is not noticeable inside the car or even on the driver's side. Even under the hood it's less noticeable.