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I have the exact same symptoms on my 2003 z71. I brought it to the dealer and to a transmission shop who tore apart the front differentials. They both said it was normal and everything was fine. I've owned it since it was new and I know it didn't always make this noise. I've posted on a few forums and everyone has said that after things get worn in a bit, they start to groan, and that theirs makes the same noises. I've got 110k on mine, how many miles on yours? I know of at least 5 other people with the same or higher mileage that said theirs also groans in auto and 4hi.