Just to reverberate what CP stated, as soon as my 2000 Tahoe and 2002 Yukon XL broke that 80-90,000 mile mark the piston slap started. It is significantly louder during the colder months of the year. Honestly mine sounds more like a diesel. Also watch the 4L60E transmission, especially with towing. I broke the sunshell gears in mine and it was approximately $2K to fix. Here is a link to a website that tells you about the common signs and symptoms of the 4L60E transmission issues.
Otherwise you have the little quirks of GM products. I seem to have to replace a power window motor about every 3-4 years. My Yukon XL is leather and needless to say GM's leather is not the best. Even with diligent maintenance (think MCOCD) and conditioning it, it still cracks and peels. Pulling at highway and interstate speeds with my boat, I have to aim for 65mph on the highway or 75mph on the interstate. If in between those speeds, the transmission is always kicking back and forth between gears in towing mode. Therefore, it kills fuel economy while pulling. On my 1 hour pull each direction, the best I can get is 10-12 mpg. Great trucks and I love mine. I still love the body design on the 2000-2006 models. Good luck with your purchase.