I have a fiesta 1999, it suddenly has developed a rattle underneath and possible centre towards the front of the car. It sounds like something rattling on a bolt. It only does this sound on roughish roads and say any speed under 30, you pick up speed the rattle cannot be heard. I have been under the car and shook everything any anything, bounced the car under weight and nothing. The only thin I can see that might be the culprit is a slight but of play at the front gearbox joint just before it enters the gearbox.
My wifes last car ( 96 fiesta ) developed a knock/rattle in it - traced it to worn anti-roll bar bushes - could never reproduce the sound by bouncing the car and was more audible at lower speeds - decided to let a mechanic do the necessary and cured it.