I had a sound too, it sounded like plastic being thrown around in the engine. It happened only on idle too... It turns out it was my timing belt. Whats worse is when you drive on it, it kills your engine. Mine, one morning, i turned the car on, and it sounded horrible, and it wasn't only at idle anymore. It turned out i messed up everything down to the bearings cause the timing was off. When your timing belt lets go, your pistons don't fire on time, and everything is off coordination. Basically, it's cheaper for me to put a new motor in the car than to fix it.
If it sounds ANYTHING like i described, DO NOT drive your car, and tow it to a mechanic and have them replace your timing belt and fix any little damage to your engine.