Sounds like debris is trapped n the fan motor blades due to mice nesting or leaves being drawn in through the air intake duct or the brushes in the fan motor are worn and slamming the glove box door jars them into contact with the commutator of the motor allowing it to operate. On many cars replacing the motor is a simple operation (some cars more complex). Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info .There is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You get a one year subscription to year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual. http://bit.ly/alldata_repair_manuals or try http://bit.ly/mitchell_repair_manuals_online and get an online repair manual subscription.