A dull whining noise from the back.

I own a 2000 Jetta GL 2.0L and I was wondering whether anyone knew what the problem was. When you first start driving, the car drives perfectly. After a while, a noise coming from the back of the car appears. It's like a whining noise that won't stop. It never happens when you first start driving. I am wondering whether it's a brake problem, or a differential problem, or something else. If anyone could help, it would be greatly appreciated.