Very common leak on all m112/113 engines (Mercedes). The usual culprit is an o-ring that the back of the reservoir uses to seal against the p/s pump. You can removed the reservoir and replace just the o-ring, or if you have higher mileage, you should replace the reservoir. The reservoir has a filter built into it and over time (usually a little over 100k) is will clog and cause your p/s to become noisy (especially under load). A good visual inspection can be done with a friend. Run the car with the cap off of the p/s reservoir, have a friend turn the steering to lock and look into the reservoir at the fluid. Is it aerated or visibly contaminated? If so, replace entire reservoir assembly with new o-ring. It's a pretty straightforward job and can be done in under an hour with the right tools.