At low speed, there is a noise emitted so pedestrians know we are around. With the windows up, you won't hear it. So if you hear it at 10+ mph, unless you have bionic hearing, this may be a symptom of a engine fault. You should peruse the topic about engine malfunction or search for whine or noise to find. This has been the precursor to a motor malfunction for some. The reason to read the posts here is that you will end up being more informed than the service advisor who typically knows nothing about our cars. Depending on your dealer, you should expect no one in the service ops who knows. They will hook up the car to the computer, read the coded messages and if a problem, revert to Germany.
If you bought a used, there should be a record of all safety recalls, annual battery inspections, and any prior problems. Get them to give you those too.
Jeff in NYC
MB '17 B250e; MB '06 E500W; MB '81 380SL