maybe an elongated strut mounting hole in the upper strut mount.
To check open the hood, on the top of the left strut tower remove the nut that is inside the metal cup over the top of the strut tower. When you remove the metal cup you'll see a nut below. Turn the wheel from full right to full left. You'll see the struts rocking in side to side in their mounts (they are supposed to rock like that) Watch carefully as the wheel is turned, if the hole is elongated as wheel reaches its steering stop the strut will slide to the other end of the hole.
If the strut mount is worn the only way to change it is to take the entire srtrut/spring assembly out of the car and disassemble the upper mount from the strut/spring assembly. If you don't have experience working with suspension springs and struts I suggest you are very careful since even with the strut fully extended the spring is still compressed about 6".
You may be able to purchase a new mount from your local ford dealer or other parts distributor. Or you can order them through
www.batinc.net