Nope, it's not the exhaust note. It seems to be coming from the fuel tank which is in the bulkhead directly behind the seats. The best I can describe the sound is it seems like the hum of snow or truck tires. It goes away when you take the car off of cruise control and accelerate or decel...????

Thought I read somewhere back the SLK has a known history of a having a loud fuel pump or something. Whatever it is it sure is annoying.