I had this issue on mine, couldnt figure it out. Once I replaced my Front lower control arms, struts, mounts and swaybar links + alignment, it was all good.
Upon inspecting the parts I removed, my guess was the swaybar links because the joints were floppy, not tight at all.
They're pretty cheap on Rockauto.com, spring for the moogs, but anything is better than nothing.
2006 Focus ZX4 SE Auto -commuter