There is one (or more) plastic shifter bushings that is causing the squeak. I had the same thing happen years ago on my W124 (and holding the lever suppressed the squeak).
The bushing(s) should be inexpensive. Had it done by the dealer then, so I can't tell you how to go about the repair...sorry.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car