Why does my '78 Buick Regal make screeching noise while driving?

When first driving their is no sound, but after about 15-20mins my car, 1978 Buick Regal, begins to make a screeching sound while driving. However the sound goes away when breaking. Any ideas?|||You need front pads and rotors machined. Think about it the stops when you apply brakes. The wear indicators are rubbing on rotors, they do not start making noise until the brake rotors heat up.|||Sounds like a slipping or worn belt.|||power steering squeels when it is low on fluid; check all fluids and belts... I had a 77, buick regal, and an 83 buick regal...they are faily easy to work on...