So,
While I was getting the vac line to the transmission sorted out, I found the following parts sitting in the underbody cover when I removed it. The black piece doesn't look important, but the round bit looks like a bushing of some sort. My guess is that it's a shifter bushing. If so this would explain the rattling I've been hearing going over small bumps in the road.

What say ye?

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1991 300D 2.5 "Rocinante"
2002 Golf TDI "Teen Spirit"
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1984 300D
1966 Mustang I6 3sp
1985 Mazda RX-7 GSLSE
1982 Toyota Supra
1977 Datsun 280z
1971 Datsun 240z

The white (now yellow/brown) thing certainly looks like the remains of a shifter bushing to my eye. If it dies, the shifter can rattle and also make a soft "clink" noise when you shift through the gears.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)