1982 300CD engine knocking noise
 

I am having a noticeable knocking sound from the engine of my '82 300CD while accelerating or revving the engine. It sounds like it is dependent on engine speed, but goes away if the engine is help at a certain RPM, only happens when engine speed increases. Check out the video, the knocking can be heard pretty clearly.


Any idea what this is? What are some troubleshooting steps I can take? I am pretty experienced with gasoline engines but still kinda new to diesels.

Thanks !