[BlazerLT]

Could be the latch and you can tighten that up by wrapping electrical tape around the loop on the lift glass to take out the play.

Also, you might want to look into the interior ventilation check valves on either side of the window at the rear upper corners of the truck.

If you open the lift glass and take a look where the gas window strut is you will see two or three philips plastic screws that you can tighten the rear plates up.

What made me think it is the rear vent valves is you saying that it got worse when you opened the windows.