Odd Knock turn the wheel to reverse back?

Hi

I have a problem with my focus estate/station wagon.

When driving normally, no out of the ordinary sounds. If I pull forwards and the turn the wheel to reverse back, I get a single knock. If I am travelling forward and need to turn the wheel to full lock, I get a single knock.

I have checked ball joints, drop links, and drive shafts but I cannot find anything loose or worn.

Could someone suggest what else I could look at that might cause this?

Many thanks

Kevin