Another Mirror Question

My drivers side power mirror has some shake in it. whn you are going down the highway it shakes bad enough that you can't see much out of but a blur, but if you just touch it with your finger it stops. It is only the glass portion that is shaking(and the frame that holds th glass), not the entire mirror assembly that bolts to the door. I have never taken one of these apart before, and wouldn't know where to begin. Is there something back there that may have come loose or could be worn. If anyone has had a similar problem and knows how to correct it let me know. Much thanks to the forum I would be broke without you all.