I have had the same issue with clunk. 2019 SLT 4X4. Was going to have it looked at on my 15k service last weekend but the transmission tech did not work on Saturday. Will have to make another appointment. Anybody had the issue looked at?
This is the one issue that I noticed before purchase. The seat is very firm compared to the Ford Lariat I was driving. After driving the Sierra for a month I don't have any problems associated with firm seat. Good to go. Love the truck and the seat!