Hello everyone!

I've an Audi B5 A4 S-line, 1.9TDi, year 2000.
In about 5 months I hear a loud cracking sound when I turn steering wheel. The noise is stronger when turning steering wheel slowly and after a long driving( I hear it more in slow speeds).
After my mechanic saw the car, he told me that soon I had to change the power steering.
Two weeks later hydraulic oil started to leak and we changed power steering(rack and pinion). The rack and pinion wasn't new but we took it from another audi a4 b5. After that the leak stopped. The noise still exists but not as loud as before. Right now there is no noise in first 20 minutes of driving. After that it gets louder the more I drive.

Do you have any idea, what the problem might be?

Any help would be appreciated.
Thanks