On my 82 300SD, I have had a nailing problem for several years.
It used to vary in intensity, but now is louder and constant in intensity. It only seems to occur at low RPM. I don't hear it above around 1000 RPM and I don't think that engine and road noise is masking it.

I have isolated it to cylinder 3. If I loosen cylinder 3's fuel line at the injector, it goes away. When I open any other cylinder's line, it increases a tad.
I have taken it to my local MB repair shop and they concur that the problem is nailing. They say it is annoying, but will not hurt anything. I have searched the web for more info, and from the little I could find, they also say that it will do no harm.
My local MB shop has also worked on other 300's with the same problem and were not able to cure their nailing either. They have replaced injectors, moved injectors and replaced pumps. The nailing remains and is constant on one cylinder.
If I run straight Lubro Moly Diesel Purge, the noise goes away (Cetane boost?). I use RedLine 85 Plus, but it doesn't seem to make any difference.

Does anyone know what causes it and what it takes to get rid of it?

The only thing I have come up with is that is some kind of pre-ignition, but I don't understand how pre-ignition could make such a sharp sound.

Wayne

82 300SD
98 ML320