One problem related to loud engine noise has been reported for the 2003 GMC Sierra. The most recently reported issues are listed below.
When at a stop, the vehicle vibrated and made noises. The vehicle was taken to the dealer three times. The dealer said it was normal. The vehicle had a rough idle and made loud noises when air conditioner was on. When turned off, it made a louder noise. The vehicle was taken to the dealer a total of five times since then.