One problem related to noises during shifting has been reported for the 1999 GMC Sierra. The most recently reported issues are listed below.
Main complaint is transmission. Took vehicle in at 6000 miles and told dealer about downshifting problem and major clunk or bumping noise after downshifting after starting from near stop. Also reported hard shifting. Dealer reports scan ecm system no trans codes and roadtested results show operating as designed. Door lock bezels falling off. Dealer replaced. This was 10/20/99. Took vehicle back in with 17102 miles on may 5/2000. Complained of rattle in pass. Door. Was reported that rear seat belt holder not fastened. Replaced guide and cover for safety belt. Rattle in pass. Door showed regulator loose from door skin. Tightened and aligned . Ask for check in noise in driver's window, going down, results were operating as designed. Ck. Bump in rear on accl. Results on checking drive train: operating as designed. Condition normal. Ck. Brakes acts as if don't want to stop times. Results were operating as designed. Still not happy with brakes! right tire found to be not round. Replaced. Taken to dealer on Nov. 8/2000. Accl. Sticking at top position. Throttle body sticking and replaced per bulletin[which I never recieved] . Had labels placed on vehocle about caliper when replacind disc brakes on vehicle. Check trans. Downshifts after coming to stop then accl. Makes tripple bump and hard shifts. Scan system found no codes: operating as designed. Trans. Still not right or shifting right and bumping! door lock bezels falling out. Checked bezela and aligned. Owner reported sounds like only one horn functioning. Low volume results showed horn operating as designed. Not happy, they seem to think I'm stupid. Repeat complaints all ignored except horn. It was fixed!Dec. 8/2000tran.