Interior Noise

[crsgmc]

I have a 1996 GMC K1500. When I turn on the internal fan, it screams for a few minutes and then goes away. This has been going on for years since I have owned the truck. I am sure that the bearings are shot. Yesterday I started the truck and there was a loud banging sound. I left the truck running and listened from the passenger side. The sound is fairly loud and it sounds like it is coming from the blower motor. The blower switch was set to off so I cranked it to high and it quit banging for a few minutes and began again. I have already ordered a new fan and motor, because I know the bearings are bad. Am I missing something? If it was turned off how could it be the motor or fan. The sound is kind of a thunk......thunk. I think I will remove the fan and motor, don't want it to get hot, or cause other damage.