So my 2006 Suburban has what feels like a warped rotor. However, it is intermittent. It happens very frequently, but not every time I brake. Sometimes it happens on flat terrain, other times when decending a hill. And sometimes it doesn't happen at all, smooth as glass. If it were a warped rotor, wouldn't it happen everytime I brake? What else could it be? My Suburban is full-time AWD, so could it be something in the drivetrain? I've done several searches, but have not found anything. Anybody have an idea as to what this could be?