2000 Kia Sephia humming noise

I have a 2000 Kia Sephia automatic with air. While driving, either on the highway or Routes I have a humming noise. It seems to be getting louder. Someone told me it could be the tires. They have about 10k miles on them and are in excellant shape.
Has anyone else had this problem with their Kia?

I have almost 66k miles on it. I also have to replace the back brakes...can anyone tell me how to do this on a Kia?
Thanks,
Barb W