This same issue has just begun with my GMC Sierra also (2018) only 1,000miles, I took it to the dealer to get it checked out the next day they called me said it was fixed that the leaf springs needed some grease or lubricant... next day noise gets even louder so I’m fixing to take it back to the dealer tomorrow morning and fighting them to get the leaf springs switched out with new ones. I’ve owned 5 different GM trucks and only my 2015 had this issue but was fixed when they removed the leaf springs and replaced with new ones. Hoping that could fix this one as well because to be honest my 2010 had less issues than these newer trucks that are supposed to be much better yet they have so many little imperfections that just makes me want to throw the whole truck away.. doesn’t even allow me to feel comfortable in my own investment. (Will update what happens once I’ve gotten the truck back from my dealership).