GMC Repair: 2000 GMC Yukon Air Conditioning HeadLiner Sensor, gmc yukon, vibration noise

Question
My 2000 Gmc Yukon has an interior airconditioning sensor about the size of a Quarter located in the headliner drivers side by the window. It makes a constant noise like a squeeky fan motor. How do I access it to repair it. can't find any diagrams showing what it looks like.Does the cover snap off or do I have to access it by droping the head liner. What's it's purpose (aoto air control)?  

Answer
Hi
Here is the EXACT GM Bulletin for your concern-
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
Customers may comment that they have a buzz or vibration noise emanating from the headliner in an area that looks like a microphone or a small speaker.

Recommendation/Instructions:
The plastic escutcheon grill, which is round and about the size of a half dollar and located just above the driver to the left, is the interior ambient temperature sensor. The sensor is used in conjunction with the automatic HVAC (C68). On occasions, this sensor has been known to make a buzz or rattle sound. The sensor uses a small fan to pull air over the sensor. This buzz noise may be intermittent.

Although it may not be possible to alleviate all of the buzz or noise from this sensor, Engineering has found that if the sensor is installed incorrectly that it may bind and cause the fan to make a buzz noise. Remove the sensor and while doing so check for any binding. Reinstall the sensor and re-test. If the noise is still present, the sensor itself may be noisy and require replacement.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

Now to replace it the headliner must drop down, just enough to get your hands in there. I have seen people tap on it and make it stop, but doing so can also break it, so thats your choice.
As stated in bulletin, it is an ambeint temp. sensor used with the Auto Air system.
I Hope all this helps.
Just suggestions from experience
Good Luck
Paul