anoying noise at start up

There is a sensor above the driver seat in the headliner with what looks like a 1.5 inch diameter speaker grill. There is an obvious fan motor sound coming from it. What is this? Some kind of a/c temperature sensor with a small fan? It is on when the car is running or in ACC. Thoroughly confused on this one...

That is a small microphone, most likely for onstar I would assume, for handsfree talking. My truck, 2000 envoy, has one too, and I know exaclty the noise your describing, but I don't have onstar.

I don't have onstar either.

that would be your inside air temp sensor GM part #15756446 and list for $64.98