Lexus4us

I am experiencing an annoying clicking noise coming from the general area of the passenger side dashboard when the "Passenger Temp" control is adjusted to a temperature BELOW the cabin temperature. When I adjust the temperature to a setting ABOVE the cabin temperature, the noise stops. Could this be the THERMISTOR (Cooler, Room Temp, Sensor) or possibly a SERVO issue? Any help or advice is appreciated. *Anyone know where the THERMISTOR is located on a 2004 ES 330?