When I turn on the fan, there is a loud knocking from the glove box area. It sounds just like someone is knocking loudly on a door. It lasts for about 10 seconds then stops. It has been doing that for a while. Now it still happens except it is not as loud. What could it be?

 

bad squirrel cage on fan motor, replace the fan

 

You could have a broken cage or the weights came off. Or maybe some coins?