My car seems perfectly quiet inside to me, but everyones perception is different. Can you record the humming sound and post it?
On another note, I had a horn replaced a few weeks ago and when I got the car back the engine was making a buzzing noise that would increase with engine RPM. I turned around and brought it back and had them listen. Of course they couldn't hear it, but I could. They then said they might hear it, but it was a normal noise. I insisted it began after they installed the horn. Finally after about 20 minutes of arguing, they agreed to put the car back on the lift and check the horn even though the insisted there was no way the horn install could have caused the noise. Turned out the technician had ordered the wrong horn and "tried to make it work", and it didn't fit right and was vibrating against something else which produced the buzzing noise. Once they took it out the buzzing stopped, and a week later when the correct horn went in everything was fine.
My point is I think the dealer hates chasing down "noises" because it can come from anywhere, and some noises are normal. But if you insist enough, with a good service advisor, he should at least look for a cause.
Good luck.