Has the fluid ever been changed? In a vehicle with that that much age it is possible that the fluid could be bad or the pump worn out. If the fluid has never been changed, try having the fluid flushed and see if that corrects your problem. If the noise is still there, most likely the pump going bad. I recommend having a Mercedes-Benz dealership or an approved service center to insure that the proper fluid is used. Mercedes-Benzs use a special synthetic power steering fluid. If the proper fluid is not used it can cause noise and pump wear.
change the filter by open the cap rmove the screw lift the shift and you will see the filter and change the fluid.the air bleed it by turning the wheel all the way to right then all the way to left few times.