One problem related to noises during shifting has been reported for the 2003 GMC Sierra. The most recently reported issues are listed below.
Transmission has "clunking" noise when vehicle shifts from second to third gear when starting and down shifting when stopping. The noise started at approximately 4000 miles and continues to progressively get louder as the vehicle continues to be driven. The transmission has not failed to operate at this time but expect it to fail sometime in the near future.