Humming from car in 100km/h speed

Hi,
I have a mazda 323 2002 model i purchased it recently. I noticed a growing humming sound (sound is pretty annoying and big) when the speed enters to 100 km/h upto 110km/h. It seems the sound is from back, but am not sure. Anybody experienced this behavior with your mazda 323?
 
Thanks for any input