Russ,

A couple of years ago my wife's '93 190E had the exact same problem. I had replaced the front flex disc on the drive shaft to resolve a clanking noise when shifting gears, and later a new noise came about that sounds like what you are describing.
I removed the right rear wheel and discovered that the Rear Sub-Frame Mount was defective. There are two mounts on each side, a front and rear. Hers was the front mount. I replaced it and the noise stopped.
You might not notice the bad mount just by looking at it, so try lifting up the rear suspension to see if there is any play.

Good Luck,

Dave