Knock sound from stop when accelerator pedal is depressed
I just bought a 2005 focus with 80,000+ miles and am very happy with it. However over the last few days , from stop, there is a knock sound on depressing the accelerator pedal . There is no similar sound once the car is moving and the pedal is subsequently depressed. It only happens from start. When the engine is off, depressing the pedal does not cause the sound.
I would appreciate suggestions on how to remedy this.