Although the Zetec tends to be a little rough at idle, you can often mitigate it.
Make sure that the engine tune is up to snuff. Especially the idle air control and throttle body. If these are even a little bit dirty, the idle speed may drop a little, enough to agrivate the roughness. Without saying, the plugs and plug wires must be sound as well.
Anything that can interfear with the motors mounts job of isolating normal engine vibration should also be checked. Old saging mounts are a posibility, but not overly likely. Air conditioning lines that are out of position and hitting the body can transmit a lot more vibration than you might expect. Anything that is very stiff that attaches to the engine that is touching the body, including in some cases wire looms.
Sometimes it helps to make sure that the motor mounts are not binding. Loosen the mounts then start the engine and load the engine gently in gear, both drive and reverse, (with you foot firmly on the brake of course) so that the mounts can center and have any stress relieved.
Worn mounts on the engine cooling fan or a fan that is out of balance can make a lot of vibration.
Sometimes, nothing seems to help. You may, however, be able to minimize it.
