help with my 1997 ford escort problem

Hi, there, my 1997 ford escort has a problem. After I start my car, and whenever I put my foot on the oil peddle, I feel a vibration form the peddle. it is not a natural vibration, the frequency of vibration drops a little while I am go upward on the slope. A "dada" sound also comes with the vibration. Anybody could tell me more about this problem?

Thanks a lot and happy new year!