If the noise is coming from over your head in the drivers seat - its a small fan that has something to do with the temp regulator. A few posts about this in the last year. Pretty normal.
Not sure what you are talking about here, are you referring to the speaker for the onstar?
I have ripped my tahoe completely down and there is no fan in the headliner aside from that little bitty thing above the driver in the headliner. It is behind a small round plastic insert that looks like it covers a speaker. It has nothing to do with the AC.
The AC is in the pass rear corner, start there by pulling the plastic around the AC unit itself and turning on the rear AC. You only have one compressor but that rear AC also has a fan, which is likely the cause of your noise. Could be as simple as piece of paper stuck down in that fan.
B