Vacuum Pump Leak
I'm assuming this question refers to the vaccuum pump and not the air pump.
I'm not familiar with the vacuum pumps used on MBZ, but if they are the same as the ones on Volvo's, yes they can leak. They are mounted on the engine block and have a rubber diaphragm inside. Over time the diaphragm cracks and crankcase oil begins to leak out the vent hole. I was able to get a repair kit that was a lot cheaper than a new pump and easy to install once you have the vacuum pump off