1995 E320 Wagon - back "thud" sound
 

When I'm slowing down approaching a stop light, I will hear this thud sound in the back of the car. The transmission is shifting down before the car does a complete stop. I can only replicate the situation a few times but it does not happen all the time, I guess it just depends on how fast or slow I'm stopping. This event presents itself when I'm in "D" but when I'm on other lower gears (three or two) I don't feel the "thud" sound when stopping either fast or slow. Start with the transmission fluid? Any ideas is greatly appreciated.

Thanks.

Non-expert here, but I'd take a look at the rear flex disc...the coupling between the drive shaft and diff.