Hi Fiesta's are noisy engine's especially the tappets. Hopefully its not the bottom end. I would go to a garage and get a mechanic to check it. I dont think they'll charge you for a diagnosis. Good luck!
Hi I wuld replace the complete throttle body, cause it could be the idle controle valve or throttle spindel, etc. It might the map sensor. If it were the HT's or coil packs it would misfiring as well. Good luck!