In the video you can see rotating the (left) drive shaft as I rotate the wheels (the front of the vehicle is held up). The knocking noise comes from the direction of the engine or from behind the engine. The drive-shaft is tight as well the steering connecting rod (at the wheel side) and  as well the vertical axle (not sure about the English terms). If I rotate the steering wheel (by lowered vehicle, so it stands on the wheels) there's also a quite similar noise, however I haven't noticed any steering-gear backlash.

 

What might be the source of the knocking sound?

Is it normal?

 

Here is the video:

 

 Vid link is dead but from your description it sounds the same as me. I changed the snub mount A arm bushes put on n a dead set kit on the sub frame. Still doing it. U have any luck?