I just picked my GLC300 a week ago and stopped at the dealer today to ask about the engine noice. I too was told it was the injectors. I had a 1989 gas MB that sounded like a diesel. I had that car for 10 years with many oil changes and it sounded the same when I sold it at 110,000 miles as it did when first bought. I used a few different grades of oil including synthetic and the noice never went away. The good news is the engine never skipped a beat.