As I take my car into the dealer for repair on the top, I might as well have them look at another issue. When I'm going highway speed (65-75mph U.S.), the automatic transmission starts to hum. If I accelerate then there's no audible noise. If I go to 80mph and let the car drift down in speed, the noise is not there (rules out tire noise). If I hold the gas so it maintains speed, then there's this loud and annoying hum.

Please help with advice so I'll be on more informed grounds with the dealer. By the way, car only has 17,000 miles since I drive 3,000 miles per year.

Appreciate any help. Thank you

 

rear differential might be causing noise
hard to tell if it the trans or rear end because the car is so small
try having the diff fluid checked and topped off before you buy one

 

If on balanced throttle it is likely the rear differential.

My experience is that SLK is somewhat pone to this "feature"