AC 'knocking' sound from vent below dash
 

I have a 2007 Escape Hybrid.
When running the AC, I hear this knocking sound coming from under the dash on the passenger side. It sounds like some kind of valve opening and closing quickly, over and over. It isn't continuous, but it will occur at least every 10 seconds. It might knock 2 or 3 times, then not at all for several seconds, then once again, and nothing for several seconds, etc.
I think I found the device making the sound. I can feel the vibrations of the knock coming from this device under the dash on the passenger side:

http://img253.imageshack.us/img253/4...peacmodule.jpg

Anyone know what could be causing this sound? It only knocks when the AC is running. The cooling power of the AC doesn't seem to be affected by this problem.
Before I shut off my vehicle, I always turn off the AC and let the vent run. The knocking will slowly stop over the course of about a minute while the vent runs. Then when the vent is blowing warmer air, I don't hear the knocking sound at all.

Any help would be great.

Thanks!