Puffing/chuffing followed by falling rocks? ' 02 Ford Expd 5.4 liter

I was driving along the highway about 65 mph and my car made a popping noise and then began loudly chuffing. I pulled over and got towed back to my house. The sound sounds like it is coming from the back of the engine, near the exhaust manifold. the sound is definitely more on the top and to the passenger side. When I turn off the car, The same area sounds like pebbles are falling in there. Any idea what is going on?