Interior Humming Sound

I just purchased a 2002 GMC Yukon Denali and it is loaded with all the bells and whistles, except the DVD system and Sunroof. I have been noticing a humming sound when I first start up the engine. It sounds like it is coming from the vent/speaker looking thing located right above the drivers seat in the headliner. When I push on the plastic vent/speaker looking thing it stops humming. I have no idea what this vent/speaker thing is used for. I looked in the manual and cannot seem to find out what it is. It is approximately an inch and a half in diameter. It isn't the air vents that are located in the headliner right above the 2nd and 3rd row seats. I don't think its a speaker, because there is no music sound coming from it when the radio is turned on. It will eventually go away as you drive the vehicle, but I have to turn the stereo up to hide this humming sound. Can anyone tell me what this vent looking thing is used for? I don't want to pry it open, because I might risk damaging it. Thanks!

The vent above the driver's left shoulder is an air intake to measure air temperature on the driver's side for the auto. climate control.

GM has a letter out addressing the exact issue that you are having with it. The letter has the vent replaced unless moving it around can fix it.

You may be able to get behind the headliner and give the vent a 1/4 turn or so, or you may be able to carefully turn the vent from the "front" with a small tool carefully placed in the holes and a slight turn.

Ask your dealer for more info. - they can give you part numbers and probably how exactly to fix it.

Hope this helps.

GMCTech...Thanks so much for your help! I will try your advice and see what happens.