If the driveshaft has been attended to, the only other thing that would make that kind of noise is bearings.

But I think it's a MB characteristic. My W124 has a pronounced rhythmic howl at low speeds...always did. I think part of it is the aggressive tire tread pattern, but mostly inherent with the car. I live with it otherwise, since the car runs fine.

My ML350 and CLK430 don't have this issue though...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car