You know the plastic under the car, under the engine that keeps dirt from entering the engine.

Well mine, there's one screw where it's head is broken, so the screw is still in the hole, now the plastic makes noises when driving because it hits under the car.

Does anyone know how I can fix this? Id like put another screw .. but the hole is already fill with the broken screw .....

most of them are some type of clip that the bolt screws into . The clips are replacable. Might try to replace the clip ...if thats it

drill, visegrips, hammer. It's not exactly a mission critical screw. Go agro on it and take no prisoners