Servo noise in warm condition at A8 d2 Bj99

Hello recently have a problem with the power steering on the A8 Bj 99 4.2. In cold condition everything is super ,sobalt man long distance or the car runs about 1-2 hours , starts to make the servo at the direct noises! Does anyone know what it could be ?? As soon as it is turned off and it cools down again, it is gone again, comes with longer but time again !Oil I have checked, container is full???????