its like turning your heat knob from cold to hot, or somewhere in b/w. . ones that came w/ the digital climate control had the air mix control motor. the computer tells the motor where to move depending on the temperature u set.
but anyways IMO its not the motor thats rattling, probably a screw that came loose and making the whole unit move. that motor is not running constantly, only every now and then.
look for loose screws. if everything is in its place, then i might be wrong and there might be a problem w/ ur climate control.
after that try messing around w/ your climate control like changning your temperature and see if you hear any different sound coming out of the motor?
and also do you hear the rattling sound when the engine is not running, i mean when the key is in "on" position.
turn the key to "on" position and turn on your fan. see if it makes any sound then, if not and it only makes sound when the engine is running, then there's definately a screw or something loose.