I suspect your front seal is leaking and dripping onto the pulley. When you start your car the oil that has leaked down the pulley and causes the belt to squeal until it burns off. The seal kit from the mecedes dealer is only $20 but it requires removal of the pump to replace it. (All outlined in a good repair manual). Don't spend big money on a new pump until you try this. If when you get it off the bearings are bad (there is play in the shaft) you can swap the pump for a rebuilt one.