Ford Fiesta - 2010 fiesta timing belt cover - sweep

Hi, I'm looking for a bit of advice. My car has had a rattle/buzzing noise, worst at 30mph in 4th gear. Goes away when you take your foot off the accelerator generally. The garage has decided they think it's the timing belt cover, but are "investigating it further". If this is the problem does anyone know if this is covered by Ford warranty. It's also a Ford direct - the car is 13 months old. thanks very much