If it knocks only when turning (occasionally when straight) it's the CV joint on your axel. Replace the whole axel basically.
The wheel bearing could be bad too but usually doesn't "knock".
The cv joint is typically the first place to look.
Dictionary.com Unabridged (v 1.1)
axel /ˈksəl/ - [ak-suhl]
noun Figure Skating.
a jump performed by a skater leaping from the front outer edge of one skate into the air to make 11/2 rotations of the body and landing on the back outer edge of the other skate.
I think I may have the same problem as you. I noticed recently that when I turn left or just hold my steering wheel a bit left of center I hear a clicking/rubbing sound from the front right wheel. At first I thought it might be the slotted rotors, but it only does it when turning one way. I guess I need to check the CV boots today.
Is replacing the axle a straight forward job, or does it require special tools and such?