Thanks for the comments.

I had recently changed my tyres from 205 to 225 on the same rims. Despite the broader profile, tyre roar was decent but bump suppression less. However, I noticed wheel bearing wear and changed all 4 bearings which eliminated the noise.

Now that the background noise from the wheel bearing and tyres were gone, I noticed increased external noise especially from the dashboard vent. It seems to particularly carry high frequency sound i.e. exhaust high pitch rasp and wind turbulence. The funny thing was that by closing the central air-vent, the sound was considerable muffled.

The w124 I drove was a 1992 260E in stock soundproofing. I agree that I may have noticed the difference only after driving it. It is not excessively loud, but again I wonder if it is worth the effort of dismantling the dash.

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1983 w126 280SE - daily drive
1983 w123 200 - sold