1998 Chevy Tahoe grinding sound

My 1998 tahoe has 159000 miles. At 55 mph when I let off the gas to slow down a loud grinding noise wil start. If I tap the gas it will stop but then start up again. When backing out of the driveway, after this started, the truck would not go into reverse at first. The same grinding noise occurred. I took it to my local mechanic, he took it apart and said that the torque convertor was shot but the transmission was absolutely fine. He put it back together and it still makes the same sound. Could it be the transfer case, maybe? I don't know, any suggestions would help.