Strange intermittent vibration/chatter/noise behind dash in '95 SC Riv

I need that sound effects guy from the GEICO commercials to reproduce the sound I am hearing behind my dashboard. I am fairly sure it is related to the dampers/regulators (or whatever they are called) for the climate control system. It is like a combinations of a semi-squeaky chatter and vibration that lasts for a few seconds at a time. There might be a series of a few of them and then it will go away - sometimes for days - sometimes for hours - sometimes for minutes. Other than the strange noise everything seems to work just fine. I typically do not use the AUTO function of my AC system. I usually just put it on 60 degrees and use the fan speed accordingly. Sometimes I will adjust the temperature, but will always control fan speed manually.