Steering Is Tight problems of the 2008 Mercedes Benz GL450

I have a 2008 Gl450 when you make a turn you'll hear a sound and it is so hard to turn the steering wheel. I took it to a Mercedes Benz mechanic and they flushed and replaced the power steering fluid, the wheel worked for a couple of days and the noise is not as loud but 2 weeks later the steering wheel is hard to turn again and the noise is loud again.