1990 e noise in rear end
 

I have an 1989 190 e 2.6. It has developed a very slight pinging noise coming from the drive train in the rear. The noise is more in sync with the motion of the car than the gear noise. It is only noticable when moderately accellerating from a stop from about 10 to 20 miles per hour. In the old days I would describe it as a u joint noise but perhaps that is "old hat"
Can someone help identify what and how expensive it might be to repair?