Clicking/thumping noise from the rear passenger side

My car started making a clicking/thumping noise from the rear passenger side this past July (2012). It almost sounds like a fan motor rubbing. At first the noise seemed to go away once I turned the rear temperature control up so it was not trying to cool. Now that our weather is turning cold the noise seems to come and go (even when the car is not running). Yesterday the car was parked (not running/no key) and I could hear the clicking from the outside of the car. It clicked for a few seconds and then stopped. I then started the car and drove for a few blocks and did not hear the noise at all. It comes and goes and I cannot pin point where the noise is actually coming from. It clicks both when trying to cool and heat.

Please help!

Thanks,
Rod