[ImperialCrown]

This is the 'pipe organ' effect. The shudder would be a very low note.
You can try to roll down another window or open the vents to break the air column or place an air deflector or Ventshade ahead of the rear window to break up the air stream before it goes past the rear window opening.

 

[chuzz]

I had the same problem with my 08 Caliber and this piece of crap 07 Equinox does the same thing. I just keep the windows up and run the a/c.

 

[tkyoshi]

It's called Wind Buffeting and it can happen on any vehicle (especially with aerodynamic designs now).It's to do with air pressure changes.

From the Owners Manual:

Wind Buffeting
Wind buffeting can be described as the perception of
pressure on the ears or a helicopter-type sound in the
ears. Your vehicle may exhibit wind buffeting with the
windows down, or the sunroof (if equipped) is in certain
open or partially open positions. This is a normal occurrence
and can be minimized. If the buffeting occurs with
the rear windows open, then open the front and rear
windows together to minimize the buffeting. If the
buffeting occurs with the sunroof open, then adjust the
sunroof opening to minimize the buffeting or open any
window.

 

[valiant67]

That's where the pop-out side windows on the back of the minivan come in handy. Eliminates this problem completely.