1999 Lumina Heater Fan Servicing

Can someone please help me? I need to know how to get to the heater fan to either repair or replace it. The fan will go for a while and not squeek when running then it will start to make noises when in positions 1,2, and 3. I turn the control to position 4 and run it for about 5-10 minutes then turn it to any other position and it will run quiet for a while. Then after a time the problem comes up again. Any suggestions? Thanks!!