If you take the vehicle up to 55 mph, will you hear the annoying hum continuously at road load?

If you let up off the pedal very slightly, so that the vehicle is barely slowing down, will the hum go away?

If you let up off the pedal completely, so that the vehicle slows down normally, will the hum change pitch or go away?

If you press down on the pedal and accelerate, will the hum be reduced or go away?

The answers to these questions may point to something other than the brakes.