Dose the noise go away after the mode has changed or does it continue all the time? Do all of the modes work normally and it the heat and cold settings as they should be? Can you make it do it any time? You may need to bring it by the shop so we can hear what it wounds like. I am suspecting a broken mode door or something like that.
It has to be a broken part in the heater and the only way to find out is to diagnose it which will probably require taking it apart to find what is broken. Give us a call if you would like us to do this work.