Grinding noise on a cold start

I have a 2009 MB C180 Kompressor BlueEFFICIENCY (W204) with a 180,000 mileage on it. Recently when I started my car in the morning. I heard a grinding noise came from the engine around 10-15 seconds noise then it went off. Sometimes in normal temperature the engine grind around 3-5 seconds too. I thought it was a starter problem then I changed it. The next day when I started it again in the morning the noise is still there. What do I have to look for? Do you ever experienced like this before? Thank You.