Hi everyone I just joined this forum and I am hoping you guys can help

here’s my issue I’ve got a 1999 W320 with an interesting issue I’ve searched all over the Internet but can’t find anyone experiencing the same issue, so I start the car in while in Park or Neutral I get a loud knocking sound and feel the clunk or knock In the car it is NOT the ENGINE, when I put the transmission in Reverse or Drive the sound and the feel of a knock disappear until the car is moving faster than I’d say 3 MPH, Has anyone ever heard of this or experienced this problem? I am assuming I’ve got a failed bearing or something inside the transmission but before I pull the tranny I figured I’d see what you guys think