Was the noise apparent before or you just noticed it because you found the W124 a lot quieter inside? It doesn't seem to make sense that noise should suddenly appear unless something between the engine bay and the cabin has fallen off. I have driven a late W124 which was 1994 I think and it was very quiet. It was a E200. I drive a W202 now also 1994 and it is not very quiet inside! I can hear the engine which is not a quiet one though the 1998 version "purrs". I can also hear the acceleration and the muffler roar as we accelerate. The muffler is not quiet too even though only the Cat is aftermarket. I can also hear the tyre roar on the rougher road surfaces. A Lexus would be a lot quieter.