I have a 06 1500 5.3 avalanche. It is acting weird, 85% of the time it runs and drives fine. The other 15% it does not respond to the throttle. It will not rev the engine the truck does not move at all. Usually this happens from a stop. It may idle and roll at 5 mph but no pedal response.Thought it was the throttle position sensor, but found out this is a drive by wire unit so I replaced the Accelerator pedal sensor, after having the transmission professionally inspected. The scan of the transmission revealed nothing, everything is in good shape. Did replace the left front brake caliper and have it tuned up since it was there. While the vehicle is trying to move there is a growling grinding sound from the behind the transfer case selection switch that stops when the trucks starts moving normally.Suggestions on where to go next. Am I looking at the selector switch? the transfer case shift motor? Any help is appreciated.