A high pitched whining sound, somewhat a squeeling warbling sound, can be as simple as a worn hard or inadequately tensioned drive belt, or as expensive as worn vanes in a vane type power steering pump if the sound is coming directly from the pump(which is more of a nuisance than something requiring immediate service). If the belt is old, its not a bad idea to replace it anyway to avoid a future breakage problem.
A growling, lower pitched sound, can be caused by low fluid level due to leakage over a long time at shaft bearings on the pump, a leak at a fitting or seal on the steering gear, or a slow hose leak; air mixed in with the fluid causes the growling sound; if you top up the fluid level and the sound continues, its a different problem.
A rebuilt pump is around $160 at Active Auto Parts, if you need it, and around $30 for a replacement belt. A person with average skills and metric tools should be able to replace the pump, which usually also requires swapping the drive pulley from the old pump to the new one, once you get the pump off the car. You would also need some MB approved power steering fluid to refill the system, which will purge itself automatically of air once it is filled with fluid.
I'm not sure why worn bushings would cause the kind of continuous sound I think you are describing, but maybe someone else knows of a property of this particular steering system that I don't; in general, continuous power steering sounds come from the steering pump, because it is the only thing that is driven continuously.