I agree it is a unique engine sound at start-up, unlike any engine I have ever had...but it is also the first direct injection Mercedes engine I have ever had. It sounds just like the loaner QX30 I had, and I believe it is pretty consistent with all of our engines.
Loaner cars of the same model you drive are always helpful for comparisons. If you had a noise or characteristic that you were unsure about (like our AC fans in speed 3 has a specific 'hum' to it. I was a little concerned about the fan motor until I drove the loaner that had the exact same noise at speed 3)