Have you ever driven over the "rumble strips" between the highway and the shoulder? The buzzing, or droning noise you'll hear is pretty close to what it sounds like when you've got coolant contamination in the transmission. The 2-3 upshift (when the torque converter clutch begins to engage), gets a little funky as well.

It can be a little different if you've got a C230 Kompressor (M271), though. I've found that the first sign of trouble can mimic a cylinder mis-fire under moderate load.

Either way, if anyone out there has a C, CLK, or E-Class MB built prior to 10/03, make sure you've got the improved radiator.