I'd check the water pump, mine did the same thing. For the longest time I had no idea where the leak was coming from, not even my local gas station could tell me, thats when I took it up to my local independent MB Shop. Cost me like $600-700, I forgot the exact amount. One question tho, Do you have any visual leaking, ie. puddles on the ground? Oh if you are leaking, and it's coming off the oil pan, thats prolly what it is.