GMC Repair: Humming noise, gmc yukon, similiar devices

Question
I have a 2005 GMC Yukon that has a humming noise comming out of a one inch device (looks like a speaker) that continues even if the radio or ac are turned off.  There are two similiar devices on the left and right area near the front windshield.  It only hums when the ignition is on  and I cannot find a way to stop it as in interferes with listening to the radio.  The sound is annoying.  I also thought it might be the on star system, but it is not activated and is not on.

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