Here's some information that may help:
One thing I've noticed on my car is when driven at around 40k or higher through corners with light throttle applied you can quite often hear some sort of "clinking / clunk " coming from the drive train. Its like something is just about to pop out and make the graunch sound.
One of the most recent occasions it did it I clutched in and tried to change gear, with resulted in a big graunched gear change.
I've also tried running the car with the front wheels of the ground to fault find but you can have the car idling in gear, when you touch the accelerator the ESC kicks in but it does make a very similar sound to the one that happens at speed.
Also, not sure if the other Focus's do it, but when you first start your car up and drive off, at 20kmh you here a small clink / thud from the drive train. Apparently this part of the ABS self test, but I have the old occasion where the noise turns into the graunch type of noise, sometimes when I take off from fresh start with a decent amount of turn on.
Hope this information useful, would really like to get to the bottom of this!