2006 Grand Marquis noise from AC
 

My Grand Marquis has a clicking noise from under the dash intermittently when the key is turn to run and at other tomes after the engine is running. It lasts for about 2 seconds and then stops. I can also hear it if I lower the temperature after the car is running with the AC on, in this care I can stop the noise if I turn the temp up a degree. The temperature control unit has been rebuilt with new O rings, and that did not stop the noise. Any ideas on what is causing the problem and how can I fix it?