does it change tone or stop when you change the temp. setting on the heat or ac controls.
this noise could be caused by the blend door actuator being stuck.
that sounds like the blend door actuator, especially since it is right behind the radio.
if you can pull the radio out, you can unplug the actuator to see if the noise stops.
do you have the dual electronic automatic climate control?
if you have manual climate control, or if the noise is coming from the left side under the dash... the dash board will have to be removed.
but if it has dual climate control and the noise is coming from the right side of the dash you can switch that one fairly easy.
there is only one blend door actuator on the left side under the dash for the non-dual ac system, and the manual says to remove the dash to replace it.