While Driving my 1987 300TD today I noticed a rattling noise, it does not change pitch or tone on acceleration and it sounds very much like a piece of plastic tube rolling over a dowel, Later on this evening I traced the sound to the Transmission hump near where the Gear selection lever goes to the transmission. The thing that seems to make this noise less apparent or completely go away is to manually shift the car from the lowest gear until it hits drive. doing this makes the sound disappear or get softer, when it goes away it goes for some time usually most of the Drive. Any ideas on what could be rattling around? I've went and checked the Exhaust system, all the pipes are on the hangers, there is no holes that I could see in the pipes.. Is there anything on the top of the transmission that could rattle like this?

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed