When listening for noise with the windows down, it is very helpful if you can find a street with very high curbs, or better yet, a sound wall. Whatever noise your car is making, it bounces off the curb or sound wall and you can hear it clearer. In narrowing down the problem, try:
different road surfaces - if the sounds goes away or is altered, then it points to the tires.
change gears (from 4th to 3rd) when driving at 45 mph - if it changes, then it's related to the engine or transmission.
put the car into neutral and rev the engine - if you hear it, then engine related.
check you tires for odd tread wear, exposed belts, etc.
while in the garage, open the hood and rev the engine and see if you hear something that sounds like the sound you hear on the road.
if the noise is consistent and can reproduced on command, then take a mbz tech for a ride and ask them what they think.
Before going to dealer, try to rule out the tires. Dealers always like to blame things on the tires.