1997 GMC Jimmy 1997 Jimmy Fan Blower starts by itself when

We have heard the blower running in our carport many times. I had always thought it was somehow tied into some type of humidity sensor, to clear the windshield, but it runs so long, it has from time to time drained the battery. I have spoken to the dealer, he said it may be a short (i dont think so) and no help there, I have resorted to pulling the fuse for the heater blower at night, and this prevents it from happening.
Incidently, when I start driving, and the vehicles gets to temperature, I hear a metalic "pop" under the dash, and I recognise this noise as a bi metalic thermostat used in other applications.
Not sure where to start searching. Because this happens about an hour after parking, and for a short time, it is hard to diagnose. Suggestions?