Unless there is some loose change dropped into the gear selector slots, it could be a shifter bushing.

Mine wore out a couple of years ago on my 300E, but it had an annoying "squeak", rather than a rattle...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car