'clunk' at 20K/H
 

My Focus has developed the strangest of noises at exactly 20K/H. There is a 'clunk' from the engine that can be felt via the excelerator pedal. It only happens after start-up and does not repeat if I drop the speed below 20 and re-accelarate. So, every time I re-start the engine and take off I get this 'clunk'. I though it could be an 'auto lock' but the car does not support that. It happens even if the car is not in gear (free rollong). Anybody got any ideas. Ford, South Africa, don't have a clue !!!!!!