I just bought a ML350 with only 45K miles on it. The Dealer sold it to me. I started hearing the same kind of sounds. Took back to the dealer and they told me I may have problems with the front right side brake. I took it back and they claim to hear no sounds twice! Then third time I drove with the mechanic and he heard it by the grace of God! Next day they called and gave me the bad news of Front Differential broken!! They won't cover it (as they claim they sold me the car as is!) and the cost is about $4200 for the replacement and parts! I hope you do not have the same problems.
The noise is in the steering column where it goes through to the engine compartment - shake it and it will rattle! My ML350 had the same problem and was repaired after part ordered from Germany. They called it a Steering bearing and it is a plastic boot-like part at the fire wall.
It is bad engineering on Mercedes part. The brake pads are smaller and do not cover the entire width of the rotor. So when the rotor wears out over time, it leaves a raised lip on the outer edge. When the old pads are replaced with new ones, they grind against the raised lip and make grinding sound as if the wheel bearing is going out.
Remedy: either replace the rotors, OR jack up and secure the vehicle, take the tire off and let the wheel turn after shifting it in to drive position. File down the rough outer lip of the rotor until flush with rest of the surface. It should solve the problem unless the wheel bearing is going out or steering column is worn due to leaking (common problem in mercedes) due to over engineering.
We had a soft scraping sound from the right-front which our mechanic told me was the break pad wear sensor on that side. This scrapes every time the wheel rotates and is audible when driving - especially in a tunnel if you wind down the window. This would explain why the frequency increases as you go faster. Brake pads were replaced at reasonable cost and all good.