Knocking when i turn the wheel

When i turn my wheel it knocks (low) throughout the whole revolution of the steeering wheel, it has been an issue for a while now but it's starting to bother me.

What do i look at first? Some possibililties?

I have check my power steering fuild its full. I can't get under the car to see the area it's coming from.

Tire rods?
tie rod bushings?
How do i know>?

Please help!