A few years ago I worked on a small ( 1L ) 3 cyl Mitsubishi diesel on a welder / generator that made a lot of noise ( nailing ) and was rough running. Loosened the injector lines until I found the bad cylinder, tightened it up, less noise. Did this a few times and the noise was completely gone.

Even though the engine was running, there was too much air in the system to open the injector much. The air was acting as a spring and it probably had a sticking delivery valve ( check valve ) in the injector pump.

The gen was then run 12 HR a day for 2 years. At this point blow by is so great it needs rebuilt but noting has blown up yet.