A bolt probly came loose, or the engine is vibrating and vibrating the pedal. Those Fords can run a long time can't they, my 86 F-150 has 310,000miles on it and still runs good.
I have noticed a light vibration coming from my gas peddle just about all the time. Almost feels like something isn't lubricated I guess is the best way to describe it.
Its got about 230,000 miles on it and its a stick shift. Should I be worried?