This seems to be pretty common with the Escalade AWD. The front end started whining then growling over time. It continued to get worse but didn't actually fail. The dealer wanted a crazy price to fix it, but I found a place to rebuild it for about $1400. The gears they took out were worn, but it seemed like it was mainly out of adjustment meaning the ring and pinon backlash was too much. This caused the awful noise.