A wuh-wuh-wuh noise that already can be heard at 2mph does not sound like a differential noise to me. I think it can be found near or in one of the rear wheels (one wheel lug that is just s little too long has the same effect).
To determine where the sound is coming from, you can drive the car while somebody is listening for the noise in the trunk...(beware not to get into trouble with someone in the trunk).

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)