Loud Clunking noise while steering at stationary

Mercedes » M-Class » 1998-2005
Question

I've had this weird loud screeching or clunking noise when steering the steering wheel, the mechanic tested every moving part at the bottom of the vehicle but didn't find the issue.

The noise happens when stationary or at really low speeds, and happens while the steering wheel is at the same position when changing the wheel direction the sound is gone and returns when steering again. 

the oil levels are normal i've changed them (every oil in the car) a week ago.

can you help me and find out the issue?