I have a 99 GMC suburban. A week ago I switched into 4 AUTO and the light flashed then went solid, but I started to hear a sloshing below me. When I switched back to 2 HI the sloshing went away. I put it in 4 AUTO again a little later and the sloshing came back, this time for good. It feels like something is dragging somewhere in the drive-train now. The sloshing is pretty silent when I accelerate, but grows louder, and seems to be slowing me down as I coast or brake.

Cars & Trucks - GMC