One problem related to transmission noise has been reported for the 1999 GMC Sierra. The most recently reported issues are listed below.
Today while trying to park my truck, I pulled into a parking spot and was going to back up, when I heard a whirring noise. I put the truck back in neutral,noise was gone, back in reverse noise has returned. No reverse. I abandoned parking and drove home. I had all forward gears. When I returned home, I put the truck in park. . . . . No park the truck rolled ahead and almost hit another vehicle. Called GMC, was told to take to dealer. Service manager said the problem was the reaction sun gear and shell internal in transmission. Very common problem. This is more than just a failed transmission, its a safety issue with no park in transmission. GMC basically told me I am s. O. L. , and would not acknowledge the problem with no park and that safety issue.