i had this on my mk1 and only happened when i reversed off a high(ish) kerb and the wish bone/subframe(i think)smacked on the kerb. i had it up on axle stands to find the only thing that had movement in was the ARB link arms, which i have found out are a very common fail on focuses. not to say that this is the case but they are so cheap(off ebay)that i changed both of them and now no noise. it has even improved the ride and smoothness of the car now as before it tended to follow over banding and lines in the road.
dead easy fix as well. the first took me 45mins, probably because i was messing about trying to unscrew the nut from the knuckle joint, but the second was a breeze as the metal is so soft and with a quick blast with the grinder had the bugger off and the new one on within 25mins.