For the record: I tried the 140 weight hypoid grease and it didn't seem to make much difference, although with increased mileage it seems to be quieting down.
Actually, I solved the problem as my elderly neighbor with a bit of a hearing problem, fell in love with the car and bought it from me.

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'76 240D-Sold
'78 240D-Sold
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet- under restoration
'86 560SL 124K Miles-Sold
'94 320E Wagon, 74K Miles-128K Miles JUNKED
'06 E350 Wagon, 84K Miles
07 SL550, 14K Miles