Mine does it as well, ever since new.
I found this at the tech pages at SAE ........
The European as well as U.S. market share of modern diesel engines has increased significantly in recent years, due to their excellent torque and performance behavior combined with low fuel consumption. The overall improved noise and vibration behavior of modern diesel engines has also contributed to this trend. Despite overall improvements in diesel engine noise and vibration, certain aspects of diesel engines continue to present significant challenges. One such issue is the presence of diesel knocking that is prevalent during cold start and warm-up conditions.
Its normal guys dont be concerned