A humming noise coming from rear of my car when going over 60kms an hour?

My car seems to make a humming noise when I go 60km or faster and the faster I go the louder it gets? What might this be?The car handles well and there are no rattles etc in the car its a 96 holden Barina Swing. Could it be the axel as it sounds like something in that area as it happens when wheels are going round at a spped of 60kms or more and the sound gets faster and louder the faster the car is driven/