modern fuel injection systems deliver idle fuel quantity while coasting. they must maintian combustion to keep the heat up in the catlytic converter for emissions purposes. Try switching the key off while coasting.. the engine will change tones and sound completely different.
A diesel will shut off fuel completely during coasting. If you turn the key off, it doesnt change tones at all, and as you coast to a stop youll hear it start clattering again as it adds fuel near idle speed. This noise is pretty hidden on an MB, but easily heard on pickup and full size truck diesels.
2003 Jetta Wagon TDI 145,000MI
1999.5 Jetta TDI 287,000MI
2001 Suzuki SV650S 26,000MI
2008 Yamaha Vino 125 11,000MI