When I added 2-cycle oil to my cars my idle actually got slightly louder. That makes sense as any motor oil is pretty low in cetane. To make a diesel engine quieter and still have complete combustion you need to increase the cetane. If your engine gets quieter by adding a low-cetane component like motor oil it means you have more unburned fuel coming out of the tailpipe. The best "additive" to increase cetane and lubricity is by far biodiesel. If that's not available I'd recommend an additive specifically designed for diesel fuel. I think cetane is a bigger concern than lubricity as ULSD is already additized to have "decent" lubricity, though it can be improved further with additives if desired, but I often find the cetane lacking in pure diesel for the altitude where I live. My cars definitely run quieter with additives or biodiesel compared to pure ULSD.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL