2002 Ford Escort

As I was sitting in the parking lot idling for roughly ten mins or so, I heard so some sort of a deep sounding but rough rattle from the car. I can't seem to pin point where it's coming from, but it sounds like it's coming from the engine. It not only does this on idle now, but if I depress the gas pedle slightly. Once I'm driving, the vehicle runs fine and the noise goes away. This began yesterday afternoon. Any ideas? I've never heard this sound before.