Driving at highway speed, the vehicle began to make a loud rattling noise that came from under the vehicle. A passerby pulled even with my car and began frantically gesturing. After a few seconds of confusion, me daughter said the she thought the man was saying "it's on fire." We pulled over to the shoulder of the road, exited quickly and realized there was fire visible in the engine area, seen through the grill. A good samaritan attempted to suppress the fire with an exstinguisher but moments later, it was fully engulfed.