Mmm, this drives me crazy as well, my mate has a 400bhp Focus RS and when he came in the car with me he freaked out, thought something was loose or falling out the engine bay. It's especially bad on wet or damp roads. I stumped up around £170 for the VWR lower engine mount and had it fitted today at AMD Essex. It didn't remove all of the wheel hop but definitely improved it a lot, and gear changes feel a lot better as well. AMD told me it was definitely the thicker Milltek pipe (I have the turbo back) banging against the 'tunnel' under hard acceleration (whatever that means)
I think that as mentioned, switching of the ESP also helps prevent the knocking.
Try the mount, you won't be disappointed.