Gotcha, very helpful. I'll pick up some seafoam and run that tomorrow.

I tried to get a video with my phone but it came out awful. But, I found an MSource video that sounds exactly the same to me. Increases with RPM's.

Mercedes W123 300TD Turbo Diesel Wagon Restoration Part 3: Diagnosing Engine Knock - YouTube

My concern is raised because in this video, the guy finds the issue to not be a fuel injector. My understanding is knocks of all kinds can be fairly analagous in sound though?