Well, i have the same transmission as you do in my 1996 E320- and from what i can tell you, I was experiencing the EXACT same problem for a month until one day, WHAM! No reverse- $4,700 later, and a BRAND new transmission, the noise is no longer there. I don't want to scare you, but it could be the trans. You can go for a complete rebuild for less than i spent, but im just anal like that. My tranny had 115,000 on the clock when she went-

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-My Garage-
1996 E320: White 118,000 miles ~Dad's Driver~
1996 E320: Green 109,000 miles ~Mom's Driver~
~My Babies~
2000 SL500: Desert Sand 96,000 miles
-Previoulsy Judge Judy's-
2008 E350 4MATIC Sport
-Flint Grey/ Ash/ Black Birds Eye Maple-