HiCustomer was any work recently done to the system? Was the fluid low at all when you checked it? Is the noise coming from the vicinity of the power steering pump?
Ok, make sure the fluid is full to the max mark on the stick. Sometimes the fluid can swirl in the reservoir and draw air if it's just slightly low. Otherwise it sounds like your pump may be failing. You could try flushing the system and it might help but if it doesn't I would say it will need a pump. Let me know if you have any questions.
A flush should cost around $100. It looks like a remanufactured pump is available from Mercedes for around $500, labor charges should be in the 2 hour range. I must apologize when I answered before I didn't think that your car had a separate reservoir for the power steering. I would replace the reservoir when I flushed the system. There is filter in the reservoir that can certainly cause the problem. The reservoir is $25 and about .5 hours to replace it. I've attached the instructions to replace the pump which also cover's the reservoir. Let me know if you have any questions.
Remove And Install Power Steering Pump
You're most welcome, good luck with it!