A common problem. Fan bearings. It's a lot of work replacing the fan motor. Suggest you try oiling them first. You can try pooping off the front rocker switch panels in the console. Behind you'll see the white blower assembly. Try drilling a large enough hole so that you can get some oil onto the shaft near the motor. Hole can be patched with duct tape for future access. Run fan. If noise goes away than problem temporarily solved. (Depends on oil thickness and bearing wear.)