I don't know if your benz has a smog pump, but my 300 E had that sound only when cold starting. Someone on the forum said it was the smog pump going bad. A little elf with a pitch fork on my shoulder told me to disconnect it and it would go away. So I did. Car runs the same and the dieseling sound is gone.
90 190 E 2.6 162K
91 300 E 117K
90 Laser RS turbo 169K
"The one who dies with the most toys wins"