If it's rattling for 30 seconds it could be a hydraulic lash adjuster (lifter), so the valve timing would bit a bit off and cause the rough running. Sometimes an oil and filter change can fix these sorts of problems, but if it's had regular on time servicing, shouldn't be so much of an issue.
I seem to remember there was a
TSB or something similar about rough idle for the first few seconds after a cold start - forgot the details, and can't remember where that came from though... might be worth asking the question of your Ford dealer though.