This is a lifter noise. A common occurence. My 07 2wd has the same noise. (135,000 miles) Check your oil level and only run full synthetic. I run Mobil 1 5W-30 with Mobil 1 Advanced filter. See if that quiets things down. Not much you can do except removing the valve cover and inspecting the lifters for looseness, then doing a valve adjustment. 3 main causes are; inadequate oils/filters, bad lifter, bent pushrod. Not worth it unless you are having more serious issues like the shift points being off or a much louder sound. (Pushrod-think roller coaster clicking.) Rent a GM Service manual from the library and decide if you want to have this taken care of at this point.
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