Humming noise from inside vehicle K13

I have an automatic transmission nissan Micra K13. When my vehicle is on drive and my foot is on the brake, I hear a humming noise(from engine) inside the vehicle cabin. When I take my foot off the brake and the vehicle starts moving, it goes away. And when I stopped the vehicles with my foot back on the brake the humming noise comes back in the cabin. I brought the vehicle to the dealership to have it checked and said to them that the noise was not there when I bought the vehicle as new a year back. They said that the noise is normal. I don't believe them. Can anyone tell me what can be the source of the noise?