It may be your intake manifold if they have the same issue that the 2.0 duratec has. They have little runner flaps (kinda like the butterfly valves on a carbeuretor) and those tend to wear out between 80-100k miles. If thats the root of the problem, dont worry about it. Like I said though, i'm not sure that a problem on the Zetecs, it may only be for the Duratecs.
ROTM June '13, PNW Crew #42, SR #245, Rally Rev #02