What is the smell coming from the air conditioner on a 2008 Mercedes Benz C300?

Is it a musty smell? What that smell is, is condensation that has built up in the heater box evaporator, then you turn on the heat or a/c and it smells musty. the dealer can peformed a ultra sonic cleaning of the evap for you to help with the smell. A good way to help this issues is to use higher blower speed settings, and also to use the 'REST' function when you leave the car, this leaves the blower motor on at a slow speed and can help eliminate the moisture build up. Hope that helps