So when I'm at idle at a stop light, I get this weird noise that seems to be coming from under the car. It sounds like a stone in a plastic bottle. It goes away when I'm drivinng or the car is moving in any way but it periodically comes back when the car is sitting there and idling. Any ideas what this might be? It's more of a nuisance right now more than anything...even though my car has been feeling a bit sluggish...but that could be just me to.