It's probably a stuffed bearing in the fan housing. It will get out of alignment, vibrate like buggery making that noise, then usually align itself after it gets up to speed. I'm not sure if they are user replaceable.
To get into the box, undo the screws, then pull apart the grommet arrangement holding the plugs in the back of the case (I think by memory you twist a screwdriver/knife between the two molded parts of that), then lift the top off and unplug the wire from the LCD display to the PCB, and you're in