Some questions about your problem:
How many miles on the car?
Does the pitch or volume change with engine speed? You can set the My View screen to show RPM.
Does the pitch or volume change with car speed?
Does the pitch or volume change with load (pressing or letting off the accelerator) while driving at about 35 mph?
Is the sound present while sitting still but with the engine running? You can force it to run in Park by pressing the accelerator.
Is there any sign of transmission fluid leaking?
Has the fluid level been checked?
You might want to post a recording of the sound.
Oops - I didn't see stolenmoments post before I posted - that may be it.